AUTO-683 Paginating while listing policies
limit defaults to 100

when unable to get from config_value. This way we do not need to make
changes in all tests depending on this config and code will work without
config
pagination tests added
setting blank dict config - thanks @cyli

since setting None causes problem in some other test using it without
setting a dict first
patching urlroot in RestAPITestMixin

since it is used by other rest test methods also
Merge pull request #421 from rackerlabs/auto-683-listing-policies-pagination

AUTO 683 listing policies pagination