GKE: Adjust the PATH to the right kubectl for gcloud

Force kubectl to the git repo version if we detect a built version,
otherwise a system version. Previously, your actual PATH influenced
what kubectl was getting picked up, which violates POLA for a dev
environment.

Along the way: Add the root install of kubectl.
Well, right now if you do a make clean the tests will fail because there won't be a kubecfg to execute, but hopefully that will be fixed soon. 

Since users can choose their own google-cloud-sdk install directory, it seems fragile to start encoding common ones here. How about replacing these two lines with $(which kubectl) which will pick it up from the gcloud install directory?