Fix ceph deployment after upgrade from Cloud 3 (bnc#899909)

proposal["deployment"]["ceph"]["elements"]["ceph-radosgw"] might not always be
a list. So use the already initialized radosgw_nodes variable which is assured
be a (possibly empty) list.

Fixes: https://bugzilla.suse.com/show_bug.cgi?id=899909
(cherry picked from commit 5696ee1c5b6cc6c84ba7bc4b5b1e084fe9cd32e0)