Automatic merge from submit-queue (batch tested with PRs 50932, 49610, 51312, 51415, 50705)

Add --request-timeout to kube-apiserver to make global request timeout configurable

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

Make the currently hard coded 60 global request timeout in apiserver configurable via a --request-timeout command line flag.

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

https://github.com/kubernetes/kubernetes/issues/51355

**Special notes for your reviewer**:

We plan to backport this to 1.6 and 1.7. To ease the backport work, we've kept this change to a minimum. After this PR we will submit an additional PR to rename `--min-request-timeout` to something that more clearly describes it's purpose, such as `--long-running-request-timeout-min`.

**Release note**:

```release-note
Add --request-timeout to kube-apiserver to make global request timeout configurable.
```