MASTER_ADVERTISE_ADDRESS is used to set the --advertise-address flag
for the apiserver. It's useful for running the apiserver behind a load
balancer.

However, if PROJECT_ID, TOKEN_URL, TOKEN_BODY, and NODE_NETWORK are
all set, the GCE VM's external IP address will be fetched and used
instead and MASTER_ADVERTISE_ADDRESS will be ignored.

Change this behavior so that MASTER_ADVERTISE_ADDRESS takes precedence
because it's more specific. We still fall back to using the VM's
external IP address if the other variables are set.

Also: Pass --ssh-user and --ssh-keyfile flags if both PROXY_SSH_USER
and MASTER_ADVERTISE_ADDRESS is set.