The import image api now supports a list of stores to import data into.
This list can be specified through a new "stores" field that has been
added to the request body.
During import stage, Glance iterates overs this list and send the data
to each store one by one.
If an invalid backend is requested by the user, an exception is raised.
If an errors occurs during verify, already pushed data is removed and
image state is unchanged.
Change-Id: Id3ac19488c0a693d7042be4a3c83f3b9f12313d0
Implements: blueprint import-multi-stores
92492cf504