Added build + test jobs in GitLab-CI and build + test + deploy jobs on
Travis-CI for CentOS 8.

Updated testing dependencies in the RPM spec to follow the new Python 2
package naming scheme that was introduced in CentOS 8: it uses
'python2-' prefix rather then 'python-'.

CentOS 8 does not provide python2-gevent and python2-greenlet packages,
so they were pushed to https://packagecloud.io/packpack/backports
repository. This repository is enabled in our build image
(packpack/packpack:el-8) by default. Those dependencies are build-time,
so nothing was changed for a user. The source RPM packages were gathered
from https://cbs.centos.org

.

Disabled app-tap/pwd.test.lua on CentOS 8 due to systemd-nss issue,
which was not worked around properly. Filed #4592 to resolved it in the
future.

Eliminated libunwind runtime dependency (and libunwind-devel build
dependency) on CentOS 8, because the base system does not provide it.
fiber.info() backtraces and printing of a backtrace after a crash will
not be available on this system. Hopefully we'll fix it in the future,
filed #4611 on this.

Closes #4543

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>
Reviewed-by: Igor Munkin <imun@tarantool.org>

Added Ubuntu 19.10 Eoan Ermine into CI.

Close #4583

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>

Cleaned up the .gitlab-ci.yml file from duplicating
code - added templates that storres the needed
configuration values for different jobs.
Also moved the static_build from 'deploy' tag to
'deploy_test' tag to be sure that tests will not
be run under high load.

Added static build using Dockerfile on Centos 7 for release
commit criteria only. Added the cleanup for cmake generating
CMakeCache.txt files and CMakeFiles directories to avoid of
cmake localy created setup failing inside the docker after
the whole tarantool path was copied into it. Added testing
into the static build, running only when RUN_TESTS environment
variable set to non empty value, used in gitlab-ci job to run
the testing after the build.

Closes #3668

Added ASAN tesing in commit process, used clang-8 for
ASAN build under debian-buster image. Added for testing
only the passing test suites, the rest of the tests
not used and will be enabled durring issue #4360. Also
fixed job for testing LTO with clang on debian-buster
in travis-ci, changed it to the same as in gitlab-ci,
changed default clang to clang-8.

Closes #4359

Current results have some tests with flaky results, which
blocks the deploy stage - decided to merge deploy stage into
test stage temporary to fix it.

Follows up #4156

Implemented GitLab CI testing process additionally to existing Travis
CI. The new testing process is added to run tests faster. It requires to
control a load of machines to avoid flaky fails on timeouts. GitLab CI
allows us to run testing on our machines.

Created 2 stages for testing and deploying packages.

The testing stage contains the following jobs that are run for all
branches:

* Debian 9 (Stretch): release/debug gcc.
* Debian 10 (Buster): release clang8 + lto.
* OSX 14 (Mojave): release.
* FreeBSD 12: release gcc.

And the following jobs that are run of long-term branches (release
branches: for now it is 1.10, 2.1 and master):

* OSX 13 (Sierra): release clang.
* OSX 14 (Mojave): release clang + lto.

The deployment stage contains the same jobs as we have in Travis CI.
They however just build tarballs and packages: don't push them to S3 and
packagecloud.

In order to run full testing on a short-term branch one can name it with
'-full-ci' suffix.

The additional manual work is needed when dependencies are changed in
.travis.mk file ('deps_debian' or 'deps_buster_clang_8' goals):

 | make GITLAB_USER=foo -f .gitlab.mk docker_bootstrap

This command pushes docker images into GitLab Registry and then they are
used in testing. Pre-built images speed up testing.

Fixes #4156