Add env var to point at a directory with fake MAAS API results

set FAKE_API_DATA to a path to a directory containing a file named 'maas-machines.json' and the status UI will use that as the machines list instead of querying a real MAAS server.

This also stubs out the Juju API client, but doesn't add any support for fake Juju data, since that's not needed for the placement screen, and doing it for the deploy screen would a bit more complicated (doing that would require being able to stub out calls to the command line juju client).