Add KEP for user namespaces support

This commit adds the high level overview I proposed in the SIG-node
meeting on Nov 2. The work divided in phases and intial support (phase 1
and 2) is disentangled from further improvements that community members
wanted to see (phase 3).

This incorporates the valuable feedback in the discussion at PR 2101,
making things as automatic as possible and adding a phase 3 for such
improvements, while it also leaves room for future improvements too.

Slides used in the Nov 2 SIG-node meeting are here:
	https://docs.google.com/presentation/d/1z4oiZ7v4DjWpZQI2kbFbI8Q6botFaA07KJYaKA-vZpg/edit#slide=id.gc6f73a04f_0_0

Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io>