This patch refactors pkg/util/mount to be more usable outside of
Kubernetes. This is done by refactoring mount.Interface to only contain
methods that are not K8s specific. Methods that are not relevant to
basic mount activities but still have OS-specific implementations are
now found in a mount.HostUtils interface.
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.
