Added multi-store support. User can now use '--backend'
option to pass desired store while creating, uploading or
importing image to speific store backend.
Added new command 'stores-info' which will return available
stores information to the user.
Related to blueprint multi-store
Change-Id: I7370094fc4ed47205b5a86a18b22aaa7b9457e5b
71bfd7bfad