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
2 jobs for alatiera/include-everywhere in 4 minutes and 15 seconds (queued for 1 minute and 42 seconds)
Status Name Job ID Coverage
passed manifest #42738


passed build fedora x86_64 #42739