[google compute] Add missing attributes/methods to create_node()

@Kami, @runseb - I would appreciate a review of this one.  This PR has the potential to break GCE users, but the `demos/gce_demo.py` and tox tests are all good, so I think it's in good shape.

Improvements to Node.  Adding missing attributes and methods to make it match the GCE API.
* Added Node attributes such as `canIpForward`, `serviceAccounts`, etc to `node['extra']`.
* Added `ex_add_access_config()`
* Added `ex_delete_access_config()`
* Added `ex_set_node_metadata()`
* Added `ex_get_serial_output()`

The newly added attributes and methods docs are covered at https://cloud.google.com/compute/docs/reference/latest/instances