All the backend work was done a while back for image volumes, so
this is effectively just plumbing the option in for volumes in
the parser logic. We do need to change the return type of the
volume parser as it only worked on spec.Mount before (which does
not have subpath support, so we'd have to pass it as an option
and parse it again) but that is cleaner than the alternative.
Fixes#20661
Signed-off-by: Matt Heon <mheon@redhat.com>
8a192c8403