Fix MailRoom not loading in Omnibus

What does this MR do and why?

!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)

MR acceptance checklist

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How to set up and validate locally

1. Set up Service Desk or Incoming Email on Omnibus. For example, in /etc/gitlab/gitlab.rb:

gitlab_rails['incoming_email_enabled'] = true
gitlab_rails['incoming_email_address'] = "stanhu+%{key}@REDACTED.com"
gitlab_rails['incoming_email_email'] = "stanhu@REDACTED.com"
gitlab_rails['incoming_email_inbox_method'] = 'microsoft_graph'
gitlab_rails['incoming_email_inbox_options'] = {
   'tenant_id': 'REDACTED',
   'client_id': 'REDACTED',
   'client_secret': 'REDACTED',
   'poll_interval': 61  # Optional
}

2. Run gitlab-ctl reconfigure.
3. See the error described in #468467 (closed).
4. Apply this patch, mail_room should start again.