Allows configuring GCE instances to use dual stack networking. This is desirable for us since we are enabling IPv6 across the infrastructure, due to occasional issues with cloud nat where large outbound traffic bursts sometimes cause connections to fail.

This is enabled via DRONE_GOOGLE_STACK_TYPE. It is IPV4_ONLY by default or can be set to IPV4_IPV6 if the subnetwork supports dual stack. I've tested this on a test instance with agents on a dual stack subnetwork. With DRONE_GOOGLE_STACK_TYPE unset, the agents come up IPV4_ONLY as before, but when setting it to IPV4_IPV6, the agents come up with a dual-stack interface.

This required updates to the google modules since it is currently using quite an old version which does not support setting the stack type.