core: Allow CPU family change for cluster with hosts

Allows changing CPU family for cluster which has all hosts in
Maintenance. Currently this was allowed only for empty cluster.

Change-Id: I7d5e53f15e917e84ed0bb99fa646a6d5bb866f8c
Bug-Url: https://bugzilla.redhat.com/1057232
Signed-off-by: Martin Perina <mperina@redhat.com>