Change module to create a single dual-nic NAT

Adding support for both multiple single-nic NATs and a single dual-nic NATs at once was going to be more trouble than it was worth.

This changes the module to provide a single NAT (in a managed instance group) that has two network interfaces:

1. Shared VPC network
2. cluster network

Requires:

• Cluster IP address (should probably be created with a google_compute_address resource
• Shared VPC Project ID
• Shared VPC Subnetwork (like 'uis-moodle-dev')

See https://gitlab.developers.cam.ac.uk/uis/moodle/moodle-cloud/deployment/-/issues/72 for its use.

Closes: #2 (closed)

Dynamically gets interface names so... closes: #1 (closed)