Replace stubs with mocks

This change updates the test cases for volume encryptors to use mocks
rather than stubs. The impetus for this change is comments on another
change (https://review.openstack.org/#/c/124791) that request new
tests to be written using mock. Mixing stubs and mocks would be
confusing so this change removes the existing stubs entirely.

Change-Id: I99ff78edcae5eaccabf4ea4caf04b8406ffa2f80
Merge "Replace stubs with mocks"