Use snapshot object in create_volume flow

Use snapshot object in create_volume flow

create_volume flow was making a call to db.snapshot_get to get
a snapshot dict and passing it onto drivers in
create_volume_from_snapshot() instead of an object. This causes
a conflict for some drivers, since create_snapshot and
create_volume_from_snapshot methods have two different snapshot
types (object vs. dict). The following patch fixes this by using
objects in create_volume flow and allows drivers to be updated
appropriately to use snapshot objects.

Change-Id: I262eaef9ac9f5f03d3f00f9a2c1fbfbd6c418676
Closes-Bug: #1428609