Update the CloudDNS check.

[lukebaumann]

If there is an upgrad available for the cluster, additional text is printed to stderr

* - There is an upgrade available for your cluster(s).

To upgrade nodes to the latest available version, run
  $ gcloud container clusters upgrade <cluster>

That text is captured by run_command_for_value and a ValueError is returned when trying to convert it to an integer.

The fix is to inspect the return_code of grep instead of the text output of wc -l.

Fixes / Features

Testing / Documentation

Testing details.

• [ y/n ] Tests pass
• [ y/n ] Appropriate changes to documentation are included in the PR