Automatic merge from submit-queue (batch tested with PRs 41583, 45117, 45123)

Implement shared PID namespace in the dockershim

**What this PR does / why we need it**: Defaults the Docker CRI to using a shared PID namespace for pods. Implements proposal in https://github.com/kubernetes/community/pull/207 tracked by #1615.

//cc @dchen1107 @vishh @timstclair 

**Special notes for your reviewer**: none

**Release note**:
```release-note
Some container runtimes share a process (PID) namespace for all containers in a pod. This will become the default for Docker in a future release of Kubernetes. You can preview this functionality if running with the CRI and Docker 1.13.1 by enabling the --experimental-docker-enable-shared-pid kubelet flag.
```