Implement basic `create` operation for instances

This wires up enough logic to allow instances to be created by node_gce: while
slightly limited in capabilities, it is sufficient to create an instances from
a stock or custom image using non-persistent local disk.

There are limitations:
 * no persistent disk support is yet available
 * networking is hard-coded to the `default` network
 * networking is hard-coded to use an ephemeral external IP

For all that, it provides mediated instance creation.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>