Update db layer to expose multiple image locations

* The DB API now exposes a 'locations' image attribute rather than
  'location'. The new field is guaranteed to be a list of zero of
  more items
* The v1 and v2 APIs only look for the first item in the list of
  locations.
* Related to bp multiple-image-locations

Change-Id: I830b383d8a8e50a01e461658fb9abe384de1a353