BZ #1104219 - Update HA Nova support for Icehouse.

https://bugzilla.redhat.com/show_bug.cgi?id=1104219

This includes making nova A/A by default.  If you wish to make it A/P, simply
set scheduler_host_subset_size=1.
Merge pull request #271 from jguiditta/ha_nova

BZ #1104219 - Update HA Nova support for Icehouse.