Add Test::Integration::Mattermost to qa:test
Add Test::Integration::Mattermost to qa:test

Part of #26; we added a new suite to gitlab-qa to test if enabling
Mattermost would break the rest of the flow.  Here we add this test to
the omnibus-gitlab pipeline.

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab