Allow shares to have multiple export locations

As discussed in the mid cycle meetup, many storage controllers actually have more than one "export location" that clients can mount to access any given share. Manila only allows storing 1 of these today, which is a bothersome limitation. I propose to change the database to support a list of export locations, and to make the associated updates to the REST API and the driver interfaces to allow drivers to advertise as many export locations as they like.

Also, drivers should be able to modify the list of export locations.