core: added event log when mixing RHLE 6 and 7 in one cluster

It was already disabled to mix this two RHELs, but there was not added a
specific message in the event log. This patch adds it, an example:

"Not possible to mix RHEL 6.x and 7.x hosts in one cluster. Tried adding RHEL -
6Server - 6.5.0.1.el6 host to a cluster with RHEL - 7Server - 7.5.0.1.el7
hosts."

Change-Id: Ia8e58e10f181b6742b88b464739ed39610ac69de
Bug-Url: https://bugzilla.redhat.com/1167827
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>