Add Gitlab CI configuration
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 https://gitlab.freedesktop.org/gstreamer/gstreamer-project/issues/29

GStreamer/gstreamer-sharp,GStreamer/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,GStreamer/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp