Automatic merge from submit-queue
Trusty: fix the 'ping' issue and fluentd-gcp issue #26379
This PR is mainly for being picking up the fix in #27016 and #27102 in trusty code, so that we can fix the issues in the release-1.2 branch for GCI. It contains two parts:
(1) Adding iptables rules to accept ICMP traffic, otherwise 'ping' from a pod does not work;
(2) Revising the code for cleaning up docker0 stuff including the bridge and iptables rules. I slightly refactor the code of starting kubelet and removing docker0 stuff before starting kubelet. The old code did it after starting kubelet but before restarting docker. I think doing it before starting kubelet is safter.
cc/ @roberthbailey @fabioy @dchen1107 @a-robinson @kubernetes/goog-image
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