Attach snapshot - driver only

Attach snapshot - driver only

This patch is a continuation of adding support for
non-disruptive backup. It provides a more efficient way
to backup an attached volume by creating a temp snapshot.
Since this is used internally for backup, the attach
snapshot interface is added in the driver only. For
drivers not implementing the attach snapshot interface,
backup will still be done using a volume.

Partial-implements blueprint non-disruptive-backup
Change-Id: I3649ef1d7c8a18f9d6ed0543d463354273d5f62a