cmd/juju/addunit.go: use the API for juju add-unit

cmd/juju/addunit.go: use the API for juju add-unit

This depends on both of my MPs:
  https://code.launchpad.net/~jameinel/juju-core/apiconn-from-name/+merge/181990
And:
  https://code.launchpad.net/~jameinel/juju-core/api-client-add-service-unit-to-spec-1216543/+merge/181992

But we only support 1 prereq at the moment.

Anyway, the change is actually quite simple, instead of
juju.NewConnFromName use juju.NewAPIConnFromName, and then use the
APIConn.Client() object to actually do the requests.

I've tested this integration with a 1.13.2 environment, and everything
works great. I also manually inspected 1.12 to ensure that the params
object already supported ToMachineSpec (it did).

With --debug I can see that it really is connecting to the API and not
to the MongoDB directly (it talks about connecting to wss://... rather
than ...:37017).

Also, it is actually *faster* to run than previously. I'm guessing it is
because we don't need the Presence code to trigger a sync of all the
existing nodes. When we start caching the API server addresses, this can
actually become quite fast (the current slowest part is still grabbing
the provider-state file and looking up the DNS name).

https://code.launchpad.net/~jameinel/juju-core/cli-api-add-unit/+merge/181997

Requires: https://code.launchpad.net/~jameinel/juju-core/api-client-add-service-unit-to-spec-1216543/+merge/181992

(do not edit description out of merge proposal)

[jameinel, 2013-08-25 13:11:09]

Please take a look.

[rog, 2013-08-25 14:13:16]

LGTM module the below.

Woo!

https://codereview.appspot.com/13212043/diff/1/cmd/juju/addunit.go
File cmd/juju/addunit.go (left):

https://codereview.appspot.com/13212043/diff/1/cmd/juju/addunit.go#oldcode102
cmd/juju/addunit.go:102: _, err = statecmd.AddServiceUnits(conn.State, params)
Can we also take this opportunity to move the relevant
function (AddServiceUnits) from state/statecmd into the API client code?

The only reason statecmd exists is so that it can be
called from both places, and now we're not calling it
from the CLI, it can move to its rightful home.

When we've migrated the entire API, we should be able
to delete statecmd entirely.

[jameinel, 2013-08-26 10:37:04]

Please take a look.