Fix Mattermost SSO setup
This is a proposed fix of #1217 (closed).
Why the problem exists
As was investigated in #1217 (closed):
Mattermost is tried to be configured before database is ready.
This happens because Chef has compilation and execution phases of recipe. Code here /files/gitlab-cookbooks/gitlab/recipes/mattermost.rb:91-101
shows that MattermostHelper.authorize_with_gitlab
is executed during compilation phase, while databases are created during execution phase.
As the result, it always fails to be configured at first run (which is common for running Docker containers).
Proposed solution
Calling MattermostHelper.authorize_with_gitlab
at execution phase of Chef recipe.