Since pkg/util/mount is going to move out of k/k, this exported constant
that is Kubernetes specific needed to move somewhere else. Made sense to
move it to pkg/volume/util.
Update GetDeviceNameFromMount in the mount interface to now take a
pluginMountDir argument, which is volume plugin dir with the global
mount path appended to it already.
This patch moves the ExecMounter found in pkg/util/mount to
pkg/volume/util/exec. This is done in preparation for pkg/util/mount to
move out of tree. This specific implemention of mount.Interface is only
used internally to K8s and does not need to move out of tree.