NetApp NFS: Do not reference dst_img_local before assignment

This patch moves the assignment of the dst_img_local variable to before the
try/finally block that references it so that the finally block will have this
variable no matter where in the try block an error is raised.

Change-Id: Iab48b46559f18a3d6b044abe4bc3e615e7fee075
Closes-Bug: 1309047
Make begin_detaching fail if volume not "in-use"

Like it's counterpart from Nova's volume-attach functionality standpoint
- reserve_volume, begin_detaching should fail if the volume is not in
  the correct state to be detached. This will prevent nova from
attempting to check and then detach, which is inherently racy.

Change-Id: Ie87eb0c9aea068affc94032ab2e53a91888d272a
Partial-bug: #1302774
Merge "Make begin_detaching fail if volume not "in-use""
Merge "NetApp NFS: Do not reference dst_img_local before assignment"