core: Add new NonOperational reason for cluster emulation mode

According to http://www.ovirt.org/Cluster_emulation_modes,

This new REASON if for host which can't join the cluster becuase they
don't comply with the Config ClusterEmulatedMachines or the clusters
emulatedMachine field which is a subset of the config

Bug-Url: https://bugzilla.redhat.com/927874
Change-Id: Ide5c2315c584462abed86024e2ca6207abe5b98a
Signed-off-by: Roy Golan <rgolan@redhat.com>