Automatic merge from submit-queue
Optional configmaps and secrets
Allow configmaps and secrets for environment variables and volume sources to be optional
Implements approved proposal c9f881b7bb
Release note:
```release-note
Volumes and environment variables populated from ConfigMap and Secret objects can now tolerate the named source object or specific keys being missing, by adding `optional: true` to the volume or environment variable source specifications.
```
43286a82c6