cmd/juju/destroymachine.go: enable 1.16 compat

cmd/juju/destroymachine.go: enable 1.16 compat

See bug #1253643

Essentially "Client.DestroyMachines" didn't exist in 1.16.3 so when we
changed 1.16.4 to support "juju destroy-machine --force" it broke
compatibility with older versions.

The code that used to be in state/state.go as State.DestroyMachines
was moved into the API implementation. Rather than restoring it
(because we don't want it to be on State), we introduce it just next
to the compatibility code. That should make it nice and quick to get
rid of all the compat stuff when we get to 1.20.

I tested this against 1.16.3 and it properly falls back and errors
when we expect it to. I'd like to have more unit-tests for it, but I
don't see a way to have the test suite's API server fake that a method
isn't available.

We'll also want to land this patch into trunk, since we need
compatibility with 1.16 there also.

https://code.launchpad.net/~jameinel/juju-core/1.16-destroy-machine-compat-1253643/+merge/196714

Requires: https://code.launchpad.net/~jameinel/juju-core/1.16-rpc-not-found/+merge/196713

(do not edit description out of merge proposal)

[jameinel, 2013-11-26 13:45:29]

Please take a look.

[dimitern, 2013-11-26 13:50:54]

LGTM

[fwereade, 2013-11-26 16:07:11]

LGTM, I think this is a good model.