Update kubectl to v1.17.11

[nebril]

This PR updates kubectl to 1.17.11

I pushed the image based on this change to docker.io/nebril/flux:pr-update-kubectl-11711-5241d530 , deployed it in my cluster and #3236 is fixed.

fixes #3236

[nebril]

Seems like Github is not happy with link checker:

Link checker output file: link-checker/out.md
site/guides/use-private-git-host/index.html
	ERROR	https://github.com/fluxcd/flux
		Too Many Requests (HTTP error 429)
site/tutorials/get-started-kustomize/index.html
	ERROR	https://github.com/weaveworks/flux-kustomize-example
		Too Many Requests (HTTP error 429)
site/references/automated-image-update/index.html
	ERROR	https://github.com/fluxcd/flux
		Too Many Requests (HTTP error 429)

[stefanprodan]

LGTM

Thanks @nebril