Make policy client methods return one value, and fix tests

The clients-return-one-value spec assumed the returned body was a dict,
but some client methods return a list. A list version of ResponseBody was
added for this case.

Partiall implements: blueprint clients-return-one-value

Change-Id: Ibd4f9c3c8082726933c969099390c6c0935b27f5
Merge "Make policy client methods return one value, and fix tests"