core: Add validation for empty compute resource or host group

Change-Id: Id26697e3e7a1b98c7e5e557cff3fd7078aecb67a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1127606
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>