state/apiserver/uniter: fix test

When authenticating, against a unit in state, it is the Entity, not the Tag that takes priority.
state/apiserver/uniter: remove panic

Remove an unnecessary panic from the uniter.

Also driveby in the usermanager to clean up the logic.
Revert "state/apiserver/uniter: fix test"

This reverts commit a118ca1214ebe6eb35d80180cb92f751a9b039e7.
Merge pull request #423 from davecheney/163-remove-panic

163 remove panic

Remove an unnecessary panic from the uniter.

Also driveby in the usermanager to clean up the logic.