Support per-pod hostnames

[mpnally]

Issue #21 describes the intended use of weights when introducing new revisions of a component. There is a similar scenario where it is useful to associate a specific hostname with a new revision. When you adjust weights, you are directing to the new revision a percentage of the normal "production" traffic that would otherwise have gone to the current revision. Before you start diverting "production" traffic, it is useful to test a new revision with test traffic. A simple way to do this is to configure the new revision so that it only takes traffic from a special test hostname. If the production traffic is directed to acme.com, you can configure the new revision to take traffic directed at test-acme.com. You can then run tests that send traffic to that host. If that works out OK, you can then change (or remove) the annotation so that normal traffic goes to the new revision, and adjust weights as described in #21.