engine: Block cluster change if network labels not supported

Network labels are supported only if setup networks is
supported by the cluster level. Therefore a host which
has network labels shouldn't be moved to a cluster which
doesn't support it, without removing the labels from the
interfaces.

Change-Id: I918905e2e5ca01726a0a647109b11e1bbeb5596a
Bug-Url: https://bugzilla.redhat.com/1063285
Signed-off-by: Moti Asayag <masayag@redhat.com>