RFC: state entities, replace globalKey() with .Tag().String()

[Tim Penhey]

Hi folks,

I have been going through much of the code in the state package looking
at the work to migrate all the collection keys to handle multiple
environments.

In the long and distant past (earlier this year), the Tag() method
returned a string.  There is a second tag like thing in the state
package called the "globalKey". This is used as a collection independent
key into annotations, presence, settings, networks and status.  It seems
that the only things we ever use for this are entities that have their
own tag types.

If we ignore actions (which I don't think should bother implementing
globalKey), then we are talking about: machine, service, unit, and network.

I propose that we replace all occurrences of the globalKey with a string
representation of the Tag for that entity.

One good part of this, is that for any element of the other collections,
we can use the existing state.FindEntity(tag names.Tag) function.

Is anyone opposed to this change?

Cheers,
Tim