state/api/agent: Entity stores a names.Tag internally
state/api/machiner: store a names.Tag internally
more string -> names.Tag
more string -> names.Tag
remove another must method
remove another must method
remove another must method
Merge pull request #145 from davecheney/111-api-params-takes-tag

params.Entity preparations

This PR is a grab bag of changes aimed at isolating all the places where we pass a string to params.Entity and convert them into taking a names.Tag. 

This PR isn't complete, there is still more work to be done before params.Entity.Tag can be made into a names.Tag, but it is a good checkpoint as it gets rid of all the mustParseXXX methods added in a previous PR.