Leverage Django Cache for caching policy check result

Summary:
Leverage Django Cache Framework instead of dumping info into Session

Motivation:

Horizon is storing the result of policy check for each panel in the Django session.

This was added in this patch: https://review.openstack.org/#/c/120790/

It contributes to session size growing and eventually hitting the cookie size limit when signed cookie session backend is used..

While testing, it was found that it contributes 34% of the session size.

Description:

There are a number of items to be implemented
1. Instead of caching the result into Session, move the cache to the Django cache
https://docs.djangoproject.com/en/1.8/topics/cache/
2. An entry in the django cache will be created for each scoped token:
- key: token_id
- value: cache policy_check
- expiration: token expiration
3. If no entry from the cache can be found, it will perform the policy check and add that in the cache

UX:
N/A

Outside Dependencies:
N/A

Requirements Update Required:
N/A Cache is available since Django14

Doc Impact:
Will have to document that deployer have to configure the Django Cache correctly.

Leverage django cache for this