state/apiserver: derive svrRoot authorizer from state.Entity

This change updates the method on `svrRoot` to derive their behaviour the their `state.Entity` field via the entity's Tag.

As an entity, and its tag, are interchangeable there is no change to the perceived behaviour of `svrRoot`. However this change will allow the logic to be shared with `apiserver/testing.FakeAuthorizer` which derives its actions from a Tag, not a `state.Entity` 

Discussion: https://lists.ubuntu.com/archives/juju-dev/2014-July/003104.html