kubectl: support GKE internal IP endpoint (#810)

[atward]

Adds support to use the internal IP address of the cluster endpoint.

I could not find any variable naming convention - let me know if this should be renamed to something else.

Closes #810

[bendory]

Can you please sign the CLA as described here?

Thanks!

[bendory]

@atward This PR is LGTM, but we need you sign the CLA prior to accepting your contribution.

[river-kang]

@atward Could you sign the CLA as @bendory requested? I sincerely need this commit for the work. Thank you!

[bendory]

Closing this PR as stale; author is unresponsive.