Test for object version hash changes

Test for object version hash changes

Use the ObjectVersionChecker fixture from oslo.versionedobjects to check
when Cinder's registered objects have a change (new remotable method,
new field, etc) that require a version bump.

Fixes some registration problems along the way:

1. CinderObject doesn't need to be registered.
2. TestObject should be registered where it's used, not globally.

Removes deps=requirements.txt from tox.ini since it's not necessary and
prevents us from pip installing oslo.versionedobjects[fixtures].

Closes-Bug: #1514926

Change-Id: Id57c56a75ea11411e9e54104165bd44b577c1485