.gitlab-ci.yml: merge .gitlab-ci-private.yml

.gitlab-ci.yml: merge .gitlab-ci-private.yml

`.gitlab-ci.yml` support conditional jobs with `only` and `except`.
And variables can be read from repo CI/CD settings as condition:

    build_samba:
      script: ...
      only:
        variables:
          - $SUPPORT_PRIVATE_TEST == 'yes'

Instead of having 2 copies of yml file, we can use this feature to
trigger private jobs only when a var like `SUPPORT_PRIVATE_TEST` is defined.

I've already added above var to our repos.
Once merged, we can remove custom CI config file in
gitlab repo settings, and remove .gitlab-ci-private.yml file from code.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon Feb 18 10:54:19 CET 2019 on sn-devel-144