Setup omnibus managed cronjob to trigger reindexing

This issue tracks the effort to create a cronjob with omnibus to trigger database index rebuilds automatically.

This should trigger a rake task (gitlab:db:reindex_auto or similar) at a set schedule. We might want to make the schedule configurable from the user perspective.

Note the rake task does not exist yet and we'd create this as a stub (no-op) if this helps to setup the cronjob (so we can later add the code to it, but it is already in place).

On GitLab.com, this cronjob should be deployed to an instance that has direct database access (no pgbouncers involved) - e.g. the same host we run database migrations on.