Automatic merge from submit-queue (batch tested with PRs 55841, 55948, 55945). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Set -w flag on all iptables calls during master startup

Lack of this flag sometimes causes iptables to return error code 4 (if
other process holds xtables lock). As a result, because of  `set -o errexit`,
whole startup script fails, leaving master in an incorrect state.

This is another occurence of (already closed) https://github.com/kubernetes/kubernetes/issues/7370

**What this PR does / why we need it**:

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:

**Special notes for your reviewer**:

**Release note**:

```release-note
Bugfix: master startup script on GCP no longer fails randomly due to concurrent iptables invocations.
```