ovirt-init-functions: run rhn_check only for RHEV-H

Currently rhn_check is being executed in ovirt-init-functions
even in oVirt Node image. This patch adds a validation to
run it only on RHEV-H images.

Change-Id: I3cb0480d0f1444fcef5175182c98a5f6030df4a6
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>