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 Job ID Name Coverage
  Manifest
passed #42813
manifest

00:01:08

passed #42772
manifest

00:01:23

 
  Build
passed #42822
build fedora x86_64

00:03:57

canceled #42773
build fedora x86_64

00:25:58