GCE node driver improvements

[HarrisonWAffel]

Issue: rancher/rancher#49681

This PR updates the GCE driver so that it can be used by Rancher. Primary changes include,

• Renames the existing GOOGLE_APPLICATION_CREDENTIALS_ENCODED_JSON environment variable to GOOGLE_AUTH_ENCODED_JSON, and no falls back to using the plain text value if the value is not base64 encoded. This is the primary change required to use this driver in Rancher.
• Introduces two new flags for building network firewall rules when exposing ports publicly and internally
  • In order to open ports publicly, the google-external-firewall-rule-prefix flag must be provided as well as one or more instances of google-open-port
  • If the google-internal-firewall-rule-prefix flag is provided, a network firewall rule will be generated to allow for nodes to communicate with one another using a set list of ports. If additional internal only ports are required, users can create additional firewall rules and specify the associated network tags when creating the VM.
  • During VM deletion these rules will be automatically cleaned up if no other VM are utilizing them
• Adds a new flag for specifying labels on VMs (which are distinct from tags)
• Updates the default image to a more recent version of ubuntu

I've tested this both in Rancher and using the CLI, I've also updated the existing unit tests and added a few more cases.

[jakefhyde]

Couple nits

[jiaqiluo]

Good Job!
LGTM 💯

[jakefhyde]

just a few nits