core: Preventing host activation when host is non responsive

The following introduces proper error messages + logic to the
VdsValidator to prevent non responsive host from being activated

Bug-Url: https://bugzilla.redhat.com/669475

Change-Id: I02846c6e1bc780f5b80a9e99c167f49d24c6bc57
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>