Sandbox mode addMachines API.

Implement the fake addMachines call, using
the error messages as returned by juju-core.

Include in the machine model some basic
container/machine number info which can
be parsed from the machine name.

Clean up the destroyMachines sandbox call.

Fix the go environment addMachines API call:
errors for specific machines are now
correctly handled (including error codes).

No QA needed.