Update image group policy for Docker 1.6 & v2 registry

[ncdc]

Once Docker 1.6 and our v2 registry are out, we'll need to have the image resources in the policy like this:

• only the registry is allowed to access images and imagestreammappings
• any client may access imagestreams, imagestreamimages, imagestreamtags
• remove any references to imagerepositorymappings

@smarterclayton @deads2k @jwforres PTAL

[ncdc]

We need to decide if consumers other than our integrated registry are allowed to create ImageStreamMappings. @smarterclayton ?

[smarterclayton]

Why do we have to decide that? Someone can create a policy if they want to enable it. For now we don't need it.

[ncdc]

Ok