Fix policy working in Blazar

Summary
========

Fix policy working in Blazar.

Problem description
================

Currently there is good base for policy in Blazar, but still we have some problems:
- when request comes to Blazar before any policy is enforced there are used params from
  ctx to be put in target. So when it comes to enforcing, it looks like a==a

we need something inspired by Keystone policies:
https://github.com/openstack/keystone/blob/master/keystone/common/controller.py#L68
https://github.com/openstack/keystone/blob/master/keystone/common/controller.py#L138

Use Cases
---------------
- TBD

Proposed change
==============

TBD

Alternatives
-----------------
TBD

Data model impact
--------------------------
TBD

REST API impact
------------------------
TBD

Security impact
----------------------
TBD

Notifications impact
----------------------------
TBD

Other end user impact
--------------------------------
TBD

Performance impact
-----------------------------
None.

Other deployer impact
--------------------------------
TBD

Developer impact
---------------------------
TBD

Dependencies
===========

TBD

Testing
======
TBD

Documentation impact
==================
TBD

References
=========

[1] https://github.com/openstack/nova-specs/blob/master/specs/pike-template.rst

Note: this form is based on the nova-specs template[1].