Added supporting logic to configure, manage and use multiple stores
of the same or different type/scheme. Added new config option
'default_backend' which will be used to specifiy default store
to which image will be stored.
Added support for file and rbd store. The default behavior is
maintained for backward compatibility.
DocImpact
Partial-Implements: bp multi-store
Change-Id: I1f2e8fa61d6dfecd8395a1f894f74ec5bcb5573c
87114c8ec7