core+ui: Support lowering cluster CPU level

This patch removes the validation of not lowering down the cluster CPU
level while VMs are running. A proper warning and confirmation window is
displayed instead.

Change-Id: I69a32299b5782e69aef8832501dc8969b6dc810c
Bug-Url: https://bugzilla.redhat.com/1006209
Signed-off-by: Oved Ourfali <oourfali@redhat.com>