fix(config/mail_room.yml): Respect sentinels_password if set

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What does this MR do and why?

fix(config/mail_room.yml): Respect sentinels_password if set

As described in this Issue, mailroom does not respect the passwords for sentinel set via sentinels_password. But as the password for sentinel is generally supported, this should also apply to mailroom

References

Issue

How to set up and validate locally

1. Configure sentinel with password
2. Patch the part for config/mail_room.yml
3. gitlab-ctl restart mailroom
4. Mailroom should now work as expected again.