Automatic merge from submit-queue. 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>.

Promote ShareProcessNamespace to beta

**What this PR does / why we need it**: The ability to configure PID namespace sharing per-pod was added as an alpha feature in 1.10. This promotes the feature to beta and makes the feature available by default.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
WIP #1615

**Special notes for your reviewer**:
/assign @yujuhong 

**Release note**:

```release-note
The PodShareProcessNamespace feature to configure PID namespace sharing within a pod has been promoted to beta.
```