multisite: fix bug during switch2containers

multisite: fix bug during switch2containers

When running the switch-to-containers playbook with multisite enabled,
the fact "rgw_instances" is only set for the node being processed
(serial: 1), the consequence of that is that the set_fact of
'rgw_instances_all' can't iterate over all rgw node in order to look up
each 'rgw_instances_host'.

Adding a condition checking whether hostvars[item]["rgw_instances_host"]
is defined fixes this issue.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1967926
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>