Add migration 16 - create the image_locations table

This migration simply creates the new image_locations table. It does
not yet use the table for anything.

Related to bp multiple-image-locations

Change-Id: I1536c9276bc807aac26849e37556b465e8b5c9eb
Add migration 17 - move image location data

Migration 17 moves image location data from images.location to
the new image_locations table. The sqlalchemy db driver has been
updated to look at this new table, but the complexity of multiple
locations is not yet exposed to the reset of the system.

Related to bp multiple-image-locations

Change-Id: I6c4c684f522d9b41b9930e7f79a4e6fb3e3f5c39
Add migration 18 - drop images.location

The image location data has been migrated out of this column, so it
is no longer useful. This migration simply drops it.

Related to bp multiple-image-locations

Change-Id: I5985c7ef41fceeb3d23063c7e9e76b3b9271ffa2