Fix policy default_rule issue

This patch fixed the following issue in policy:

    1. Default_rule wasn't set correctly when using enforcer.
    2. When overwrite=True set_rule method doesn't work.
    3. Add type check in __missing__ of dict and return
       the correct value of default_rule.
    4. Partially refactor the test_policy code based on the
       related change.

Fixed #bug 1202771

Change-Id: I9be1ac8bdc995adae201e9b45ee124dd525e4822
Merge "Fix policy default_rule issue"