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