Refactor of debops.gitlab_ci role

GitLab CI role has been redesigned to use new YAML syntax in tasks, use
fewer tasks by dropping most of the "testing" and have less upkeep
requirements.

Due to a https://gitlab.com/gitlab-org/gitlab-ci/issues/36 bug in
version 5.0 of GitLab CI that version is not installable at this point.
Because of that I decided to use this rewrite to reorder some files,
among other things, and this iteration of 'debops.gitlab_ci' is
incompatible with the previous version - I advise you to re-install the
application on a fresh instance and use the database backup to recover
your data.

At the moment old GitLab CI Runners might not be compatible, newer
version should be available in 'debops.gitlab_ci_runner' role shortly.

Most important changes:

- GitLab CI is now installed in '/var/local/gitlab-ci/' instead of
  '/srv/users/gitlab-ci/' directory. Home directory variable is now
  exposed in role defaults and can easily be changed if needed.

- 'debops.gitlab_ci' will install GitLab CI 5.2 by default.

- Role better recognizes updates in the git tree and handles them
  correctly. Requirement to check hashes of configuration files to
  detect changes has been removed, and future changes to the
  configuration files, if needed, will be achieved using
  'version_compare()' Ansible filter.

- Default port used by unicorn has been changed to '18083' and can now
  be configured from role defaults if needed.

- Some internal variables related to git repository addresses and
  locations of the files on disk have been rearranged and simplified.