Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

gce: Prefer MASTER_ADVERTISE_ADDRESS in apiserver setup

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: Move the setting of --ssh-user and --ssh-keyfile based on
PROXY_SSH_USER) to a top-level block because this is common to all
codepaths.

```release-note
NONE
```