DB API and models for supporting Compute Hosts Reservations

As per our design doc for Hosts reservations [1], Climate needs to be
added three new models called ComputeHost, ComputeHostExtraCapabilities
and ComputeHostReservation

 * ComputeHost will store all info concerning hosts managed by Climate
   and will keep tracability on if they are free for a lease or not
 * ComputeHostExtraCapabilities will allow admin user to add extra info
   for each host to add to Climate like GPU or SSD which are not yet
   visible thru Nova-Compute
 * ComputeHostReservation will store all reservation details when
   creating a lease, needed by Manager when starting the leases

Implements bp:host-db-api

[1] : https://docs.google.com/drawings/d/1-_DOB65LsSGR4JUaPN525bWAg1yhvEv3KNcHfmuLvYw/edit

Change-Id: Ib8f289064a9621372908be1d44db92f3719b24e1