Support signurl using compute engine service account

Hi! I noticed in #968 there was functionality added to create signed URLs without a local private key. That's awesome! I'm trying to use this and I could use a little help.

I'm running on a compute instance and I'm able to do things like list the contents of my bucket without setting up any special configuration using the default service account, but trying to create a signed URL results in this message:

```
$ gsutil signurl -d 10m -u gs://[snip]/[snip].zip
URL HTTP Method Expiration Signed URL
CommandException: Cannot get service account email id for the given credential type.
```

I'm guessing I need to set up a boto config, though I haven't had to do that for other actions. Is there anything you could suggest?

I'm using gsutil 4.49.

Thank you!