Fix MailRoom not loading in Omnibus

What does this MR do and why?

This backports !157339 (merged) to 17-1-stable-ee.

!155546 (merged) introduced Gitlab::Redis::ConfigGenerator. Omnibus has a tight coupling with the Rails code because:

1. It loads config/mail_room.yml, which is actually an ERB file.
2. This then loads lib/gitlab/mail_room, which requires code inside lib/gitlab/redis.

However, since MailRoom doesn't start Rails, startup can fail if not all the required modules are loaded.

As a short-term fix, load redis/config_generator to fix this issue.

Relates to #468467 (closed)

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