Automatic merge from submit-queue
pin GCI version to milestone 52
This is mainly for pinning the 1.2 branch to GCI milestone 52
which contains correct docker and kubelet built in.
Doing this allows us to upgrade docker to v1.11 (issue #26455)
in GCI 53 without breaking the 1.2 release branch.
@kubernetes/goog-image @dchen1107 @roberthbailey @andyzheng0831