Use GitLab SSO to restrict Mattermost team creation
Per community requests to restrict team creation (gitlab-mattermost#7 (closed)), propose omnibus adds automation to set up GitLab SSO and disable email sign-up via instructions at: http://forum.mattermost.org/t/how-is-gitlab-sso-configured-in-gitlab-mattermost/147/4
Requires Mattermost v0.7.0-rc1 or later: !451 (merged)