Allow managing policy files in keystone

Current state

the default policy rules of keystone is always used.

Desired state

A user should be able to configure keystone's policy rules.

Desired approach

As in !237 (merged) with the addition of also changing the ci/master_integration_tests/deploy/keystone.yaml and adding the default policy file to yaook/op/keystone/templates/policy.yaml (both were was done as part of !250 (merged) for cinder).

Related: #116 (closed), #129 (closed)