Derive context is_admin from policy

Use a rule in the policy.json to correctly derive a user's admin-ness
based on their roles, when it is not explicitly specified in the
RequestContext constructor.

Keystone auth_token tests X-Roles based on the user role membership
for the tenant the token is scoped to, so this will allow us to define
per-tenant admins (the admin everywhere issue described in keystone
bug #968696 won't apply here, that is specific to keystone).

Remove the admin_role config option, which doensn't do anything, and
now this should be specified via policy.json.

Change-Id: I1921a2b515ecc8ca5d37205e3cdb6b7b8695b1ac
bluprint: request-scoping-policy
Remove misleading docstrings in ContextMiddleware

Most of this information is completely wrong, so remove it

Change-Id: I00cc4eae20447c402f1451fe9aa7d72877f2c36c
Merge "Derive context is_admin from policy"
Merge "Remove misleading docstrings in ContextMiddleware"