Introduce Test::Integration::Mattermost
This environment scenario configures a gitlab container with embedded mattermost server from omnibus-gitlab[1], and runs the standard user test suite Test::Instance from gitlab-{ce,ee}/qa
We use the newly exposed #environment and #network_aliases settings to start the mattermost instance and allow it to find itself via DNS[1]
Additionally we add jobs to the pipeline to run this for CE and EE streams as ce:mattermost and ee:mattermost
[1] https://docs.gitlab.com/omnibus/gitlab-mattermost/#getting-started
Towards #26 (closed), this is a reduced scope version of !80 (merged) that should deliver the value of a new environment scenario, until a more specific user scenario can be developed.
Also includes a new #omnibus_config
accessor to make the code read a little better.
/cc @marin