Fix metaclasses assignment

'metaclass' with 'six' module should be added only using decorator for 'target'-class,
instead of attr assignment else 'target'-class won't see our custom metaclass.

this is fix after merged following commit:
https://github.com/stackforge/manila/commit/8c1a574de3e2e1b0da5f8ffa509284c84ca03264

Also added basic unittests.

Change-Id: I27cdddf32ec81c72a5f71ce8dca04f643a425b67
Closes-Bug: #1343207