Update version to 1.6.0

Merge pull request kubernetes#2479 from justinsb/tag_160_beta1

Update tests for version bump

Update makefile to 1.6.0-alpha.2

Merge pull request kubernetes#2238 from justinsb/release_160_alpha1

Version updates for 1.6.0-alpha.1

Tag release 1.5.3

Undo the cherry pick of the kubelet version bump, which should not have
been on this branch.

We'll skip 1.5.2

Release 1.5.2

Tag 1.5.2-beta.2

Tag release 1.5.1

Tag 1.5.0

Merge pull request kubernetes#1755 from justinsb/mark_beta2

Mark beta2