Move role assignment to DocumentedRuleDefault
Move role assignment to DocumentedRuleDefault

A new policy class was introduce that requires
additional parameters when defining policy objects.

This patch switches our role assignment policy object to
the policy.DocumentedRuleDefault and fills the
required policy parameters as needed.

Change-Id: I6923a191f1c7f5856eee0b282309332895abb3ee
Partially-Implements: bp policy-docs

openstack/keystone,mahak/keystone,mahak/keystone,rajalokan/keystone,rajalokan/keystone,openstack/keystone,mahak/keystone,openstack/keystone,rajalokan/keystone