Implement Nova filter for Host Reservation Pools

There is need for a Climate Scheduler filter in Nova, as
we need to make sure the hosts in the pools won't be scheduled
by other queries than Climate ones.
We also need to make sure that the user has enough rights for
asking this specific pool

See [1] for adding a custom filter thanks to Nova.conf

Implements bp:host-filter

[1]
http://docs.openstack.org/developer/nova/devref/filter_scheduler.html

Change-Id: I069b8525a6d85ace07ba86a23e54b76b33d533e7