core: disable the mixing of RHEL6 and RHEL7 in one cluster

This patch handles the special case, that the RHEL6 and RHEL7 can not be mixed
in one cluster even the emulated machines are the same.

If the newly added host is not compatible with the rest of the hosts, it is
turned to be non operational.

Change-Id: Ieb6d41b1921a6caf946cba44a6c92ef86fef6bc8
Bug-Url: https://bugzilla.redhat.com/1150191
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>