fix for lp:1212538: cmd/juju: deploy --to a non existent machine fails too late in the process

https://bugs.launchpad.net/juju-core/+bug/1212538.

There was already a test for the behaviour of deploying to a machine that didn't exist. But it was passing for the wrong reasons.

Currently deploying to a machine that doesn't exists still creates the service in the environment - but it won't be deployed- even when the correct machine id is added.

This fix will prevent a service being deployed if the machine doesn't exist.

Only applies if using the "--to" flag