core: Fix issue with old hosts connecting to new Cluster/Data Center

Data Centers/Clusters with supported version of 3.4 can not work with
3.3 VDSM. However, such hosts are stuck in unassigned mode and can not
be removed. This patch moves these hosts to ERROR and allow users to
fix them/remove such hosts.

Change-Id: I790041c12b4701d2ccce40975437fbe309fc6797
Bug-Url: https://bugzilla.redhat.com/1052314
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>