dashboard/buildlet: fix start-up crash when TLS attributes aren't set

I had never run the 44d7ecb402b7c on GCE, and never caught that it
crashed on start-up if TLS attributes weren't defined.

Updated to use the new NotDefinedError from the metadata package in
https://code-review.googlesource.com/#/c/1790/

Change-Id: Iaec8df126e4cef8026c930e8cc0163ae088affb3
Reviewed-on: https://go-review.googlesource.com/2736
Reviewed-by: Burcu Dogan <jbd@google.com>
1 file changed