Allow managing policy files in neutron

Current state

the default policy rules of neutron is always used.

Desired state

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

Desired approach

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

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