Add cordon/uncordon fencing agent

[jfriesse]

Idea is to use infrastructure of existing Clusterlabs fencing agents (https://github.com/ClusterLabs/fence-agents) and add new one which supports cordon/uncordon.

This will need to add kubectl in the fencing agent image.

[ingvagabund]

Would be interesting to actually see how much time the cordon operation takes for nodes with a lot of pods.

[jfriesse]

With fencing being handled by ansible I don't think we need agent (it was there to fit kordon into configuration) - simple calling kubectl does same job and that is perfectly doable from ansible.