Fix retyping volume that has volume type None

Modified volume_types_diff to work when one volume type is None.

Fixed _fix_encryption_specs so that the new dictionary is returned.

Change-Id: Iaba3032317f4d0649c917fb92403c4e9146fe3e9
Closes-Bug: #1311058
(cherry picked from commit 8ce85e4fd40e9def337daadfdc92aca7b064efff)
Merge "Fix retyping volume that has volume type None" into stable/icehouse