Added ability to remove given in options package from S3. TO remove the
needed package need to set '-r=<package name with version>' option,
like:
  ./tools/update_repo.sh -o=<OS> -d=<DIST> -b=<S3 repo> \
    -r=tarantool-2.2.2.0
it will remove all found appropriate source and binaries packages from
the given S3 repository, also the meta files will be corrected there.

Close #4839

Added instructions on 'product' option with examples.

Part of #4839

Found that modules may have only binaries packages w/o sources
packages. Script changed to be able to work with only binaries
either sources packages.

Part of #4839

Added cleanup functionality for the meta files.
Script may have the following situations:

 - package files removed at S3, but it still registered:
   Script stores and registers the new packages at S3 and
   removes all the other registered blocks for the sames
   files in meta files.

 - package files already exists at S3 with the same hashes:
   Script passes it with warning message.

 - package files already exists at S3 with the old hashes:
   Script fails w/o force flag, otherwise it stores and
   registers the new packages at S3 and removes all the other
   registered blocks for the sames files in meta files.

Added '-s|skip_errors' option flag to skip errors on changed
packages to avoid of exits on script run.

Part of #4839

Fixed OSX host setup for Tarantool build:
- set brew installation from Homebrew repository instructions;
- set in use Python 2 latest commit from tapped local formula,
  since Python 2 is EOL, also removed extra pip installation with
  get-pip script, because tapped formula installs pip itself.
  python@2 was deleted from homebrew/core in commit 028f11f9e:
    python@2: delete (https://github.com/Homebrew/homebrew-core/issues/49796)
    EOL 1 January 2020.
  Tapped formula created from the latest formula before its removal:
    git -C "$(brew --repo homebrew/core)" show 028f11f9e^:Formula/python@2.rb
- added upgrade packages call to avoid of fails on already
  installed packages, but with previous version;
- fixed the gitlab-ci configuration for sudo on testing hosts and removed pip
  option '--user' to avoid of use the users paths with special setup for it.

Fixed OSX host setup for Tarantool test:
- set maximum processes limit value to 2500 for testing process;
- new Mac machines are going to be added into CI and usernames on them
  are long according to internal policies. It makes a home directory to
  be long too and so a path to a unix socket created during testing can
  be longer then UNIX_PATH_MAX=108 constant which is described as issue
    https://github.com/tarantool/tarantool/issues/4634
  To avoid of it the short working directory for testing set by option:
    --vardir /tmp/tnt

Our S3 based repositories now reflect packagecloud.io repositories
structure.

It will allow us to migrate from packagecloud.io w/o much complicating
redirection rules on a web server serving download.tarantool.org.

Deploy source packages (*.src.rpm) into separate 'SRPM' repository
like packagecloud.io does.

Changed repository signing key from its subkey to public and moved it
to gitlab-ci environment.

Follows up #3380

Found that on 19.02.2020 APT repositories with packages
for Ubuntu 18.10 Cosmic were removed from Ubuntu archive:

 E: The repository 'http://security.ubuntu.com/ubuntu cosmic-security Release' does not have a Release file.
 E: The repository 'http://archive.ubuntu.com/ubuntu cosmic Release' does not have a Release file.
 E: The repository 'http://archive.ubuntu.com/ubuntu cosmic-updates Release' does not have a Release file.
 E: The repository 'http://archive.ubuntu.com/ubuntu cosmic-backports Release' does not have a Release file.

Also found the half a year old message about Ubuntu 18.10
Cosmic EOL:
 https://fridge.ubuntu.com/2019/07/19/ubuntu-18-10-cosmic-cuttlefish-end-of-life-reached-on-july-18-2019/

Removed the Ubuntu 18.10 Cosmic from gitlab-ci and
travis-ci testings.

We're going to use S3 compatible storage for Deb and RPM repositories
instead of packagecloud.io service. The main reason is that
packagecloud.io provides a limited amount of storage, which is not
enough for keeping all packages (w/o regular pruning of old versions).

Note: At the moment packages are still pushed to packagecloud.io from
Travis-CI. Disabling this is out of scope of this patch.

This patch implements saving of packages on an S3 compatible storage and
regeneration of a repository metadata.

The layout is a bit different from one we have on packagecloud.io.

packagecloud.io:

 | - 1.10
 | - 2.1
 | - 2.2
 | - ...

S3 compatible storage:

 | - live
 |   - 1.10
 |   - 2.1
 |   - 2.2
 |   - ...
 | - release
 |   - 1.10
 |   - 2.1
 |   - 2.2
 |   - ...

Both 'live' and 'release' repositories track release branches (named as
<major>.<minor>) and master branch. The difference is that 'live' is
updated on every push, but 'release' is only for tagged versions
(<major>.<minor>.<patch>.0).

Packages are also built on '*-full-ci' branches, but only for testing
purposes: they don't pushed anywhere.

The core logic is in the tools/update_repo.sh script, which implements
the following flow:

- create metadata for new packages
- fetch relevant metadata from the S3 storage
- push new packages to the S3 storage
- merge and push the updated metadata to the S3 storage

The script uses 'createrepo' for RPM repositories and 'reprepro' for Deb
repositories.

Closes #3380