dashboard: support --limit execution with rgw

dashboard: support --limit execution with rgw

When the following conditions are met:

- rgw is deployed,
- dashboard is deployed,
- playbook is called with --limit,
- a node being processed is collocated on either a mon or mgr.

The playbook fails because `rgw_instances` is undefined.
The idea here is to make sure this variable is always defined.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2063029
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>