dashboard: fix rgw user creation

dashboard: fix rgw user creation

When deploying dashboard in a cluster with rgw multisite deployed.
Due to the last rgw multisite refactor, we now expect the variable
`rgw_zonemaster` to be defined in the dict `rgw_instances`.
The idea here is to create that user on the cluster as soon as we have 1
`rgw_zonemaster` set to `true` in `rgw_instances`.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1964995
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>