support multiple resources per workspace

[pmrowla]

The one thing to note now is that since the init'ed working_dir is always used (instead of separate dirs per resource name), this class is currently only useful for managing a single iterative-machine resource at a time (since we just rewrite the entire main.tf.json any time a config change is made).

A proper TPI release should parse the entire existing main.tf.json and only add/modify/delete the appropriate named resource entries so that you can actually manage multiple resources at a time.

Originally posted by @pmrowla in #17 (comment)