stats: return HA agent score when applicable

For Hosted Engine, the engine needs to know the HA score assigned to
each host by the HA agent--if it's installed.  This patch returns the
score via the statistics interface.

Change-Id: Ibdad8accb0660dbdcc8b6e1b83f491c2f89c2d30
Signed-off-by: Greg Padgett <gpadgett@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20126
Reviewed-by: Oved Ourfali <oourfali@redhat.com>
Reviewed-by: Sandro Bonazzola <sbonazzo@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>