[r=dimitern] cmd/juju: deploy using the API + 1.16 compat.
juju deploy starts to use the API only, if possible.
Older API servers, not supporting charm uploads are
detected and the old 1.16-compatible code with a
direct state connection is executed.
In addition to that, bug #1216830 is fixed (no juju.Conn
usage in state/apiserver antmore), which required some
refactoring, namely moving conn.DeployService and also
conn.AddUnits out of juju.Conn and into a juju/deploy.go,
taking a *state.State as argument.
https://codereview.appspot.com/44240045/
R=rogpeppe