Create default revision tag automatically

[dgn]

istioctl creates a default revision tag automatically when you deploy Istio into the cluster, even if your instance has revision name set. We should look at doing the same with an IstioRevisionTag.

Options we discussed:

1. Create an actual IstioRevisionTag in the cluster when creating the first IstioRevision
2. Create an implicit/virtual revision tag when creating the first IstioRevision