core: Allow multiple fencing alerts for the same host

Allow following alerts ussed in fencing flow to exist in db (and be
displayed to user) multiple times for each host:

  VDS_ALERT_FENCE_OPERATION_SKIPPED_BROKEN_CONNECTIVITY
  VDS_ALERT_NOT_RESTARTED_DUE_TO_POLICY
  VDS_ALERT_FENCE_DISABLED_BY_CLUSTER_POLICY
  FENCING_DISABLED_IN_CLUSTER_POLICY

Change-Id: Ic1b13cfac9d8b3469a6458e13691fbfde197ec80
Bug-Url: https://bugzilla.redhat.com/1142904
Signed-off-by: Martin Perina <mperina@redhat.com>