engine: HA VM Reservation feature

Adding HA VM Reservation feature,
The HA VM reservation feature ensure the safety of
HA VMs in case of host failover,
without negatively impacting performance.

* Added a checkbox to the Cluster popup
* A new weight function - scoring all hosts in the cluster according to there HA VM count.
* A new balance function - migrating HA VMs from over utilized host to underutilized ones.
* Rest api updated - adding a new property to the cluster object "ha_reservation".
* A new scheduled task - monitoring the cluster HA state.
* A new alert - alert when a cluster state is not HA failover safe.

Feature page: http://www.ovirt.org/Features/HA_VM_reservation

Change-Id: I44e698e58ff5e4a0b74249d3fe3cf4acf042c324
Bug-Url: https://bugzilla.redhat.com/1036753
Signed-off-by: Kobi Ianko <kobi@redhat.com>