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 <[email protected]>