build: add CentOS 8 into CI / CD

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>
(cherry picked from commit e3d9d8c9)