Add Glance policy tests

Glance supports granular permission that is specified by a policy file.
See http://docs.openstack.org/developer/glance/policies.html for more details.
Tempest should configure the policy file to some non-default policy and test permissions.
Example:

Edit /etc/glance/policy.json

{
    "default": [],
    "add_image": ["role:admin"],
    "modify_image": ["role:admin"],
    "delete_image": ["role:admin"]
}

1. upload an image using a non-admin and a non-superuser role

2. Try to delete using 'glance image-delete' it with a user which is not in an admin role.

Non-admin fails with HTTP403, admin succeeds