engine: added EvenGuestDistribution policy

The new policy should make sure that the VMs are evenly
distributed among all host in the cluster so every host
runs approximately the same number of VMs.

Change-Id: I2f2036dfc8a410c787a195d7d56ac3ed4ace81a7
Bug-Url: https://bugzilla.redhat.com/1036749
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>