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

[erjohnso]

@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

[erjohnso]

FYI - I also tested using this branch with the GCE drivers for both ansible and salt-cloud, so I'm feeling good about not breaking folks.

[erjohnso]

When I made the last two minor changes, it resulted in a merge conflict. I made a minor edit to allow a clean merge and rebased.

[erjohnso]

Added a snippet to demos/gce_demo.py to show how to use the new multi-disks param when creating nodes. Once travis passes, I plan to go ahead and merge this.