[gke.py] Fix output with Kubernetes contexts.
[gke.py] Fix output with Kubernetes contexts.

The "check_output" command could occasionally be called without ensuring
that the appropriate Kubernetes credentials were available due to its
dual-purpose use as a bootstrap to check the credentials and a tool
to capture output from a credentialed Kubernetes command.

Remove this dual-purpose functionality. Checking credentials will now
directly call the top-level "check_output", and any current
Kubernetes tool "check_output" invocation will always include
credentials.

TBR=iannucci@chromium.org
BUG=None
TEST=local

Change-Id: Ie1634e5b5d4862423598ebffad90b7c8ec6a7a9f
Reviewed-on: https://chromium-review.googlesource.com/706795
Reviewed-by: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>
Commit-Queue: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py