Restrict users from downloading protected image

https://review.openstack.org/98737
commit 0656386e99b0c9ed5b5ba4a1e17f0b8d7d50aad6
Author: Abhishek Kekane <email address hidden>
Date: Wed Jun 4 13:55:06 2014 +0000

    Restrict users from downloading protected image

    Added new rule in policy.json and applied that rule to
    'download_image' policy.

    For example,
    "restricted": "not ('test_key':(test_key)s and role:_member_)"
    "download_image": "role:admin or rule:restricted"

    So if 'download_image' policy is enforced then in above case only admin or
    user who satisfies rule 'restricted' will able to download image. Other users
    will not be able to download the image and will get 403 Forbidden response.

    In addition, delete property access should be restricted for other users
    so that they will not be able to delete the property of the image.

    [test_key]
    create = admin,member
    read = admin,member,_member_
    update = admin,member
    delete = admin,member

    Added new method to create dictionary-like mashup of image core and custom
    properties.
    Modified v1 and v2 api to add download restriction.
    Modified logic of caching to restrict download for v1 and v2 api.

    DocImpact:
    Need to add new rule in policy.json
    "restricted": "not ('test_key':%(test_key)s and role:_member_)"

    blueprint: restrict-downloading-images-protected-properties

    Change-Id: I05bad0441952150bd15b831ac1b1a0bb9ae79c74