Keystone v3-ext/OS-ENDPOINT-POLICY endpoints implementation.

Existing policy client is missing policy association API calls
for the endpoints, services and regions. These are supported API,
as mentioned on the wiki given below:

https://developer.openstack.org/api-ref/identity/v3-ext/index.html#associate-policy-and-endpoint

Closes-Bug: #1682641
Change-Id: I4af1e4862a17216d65446e8c29bd1b886f5d8c24
3 files changed