core: adding HA score with proper scheduling policies

This patch adds the HA score to VdsStatistics, filling it up where
relevant, and using it when scheduling the hosted engine VM.

Change-Id: I24879ed64cab829969556fd71786b32d3aaaf0c7
Bug-Url: https://bugzilla.redhat.com/978268
Signed-off-by: Oved Ourfali <oourfali@redhat.com>