Add Gitlab CI configuration

This commit adds a .gitlab-ci.yml file, which uses a feature
to fetch the config from a centralized repository. The intent is
to have all the gstreamer modules use the same configuration.

The configuration is currently hosted at the gst-ci repository
under the gitlab/ci_template.yml path.

Part of gstreamer/gstreamer-project#29
4 jobs for alatiera/include-everywhere in 5 minutes and 6 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Manifest
passed manifest #42813

00:01:08

passed manifest #42772

00:01:23

 
  Build
passed build fedora x86_64 #42822

00:03:57

canceled build fedora x86_64 #42773

00:25:58