Automatic merge from submit-queue

Add subPath to mount a child dir or file of a volumeMount

Allow users to specify a subPath in Container.volumeMounts so they can use a single volume for many mounts instead of creating many volumes. For instance, a user can now use a single PersistentVolume to store the Mysql database and the document root of an Apache server of a LAMP stack pod by mapping them to different subPaths in this single volume.

Also solves https://github.com/kubernetes/kubernetes/issues/20466.