Add .gitlab-ci.yml file, and define build jobs in groups.

Once gitlab-runner set up, builds and tests can be triggered
automatically in parallel when push to gitlab.
Also, with gitlab-runner autoscale mode, build instances
will be created and removed on demand.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>

Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Tue Oct 31 15:32:16 CET 2017 on sn-devel-144