Add list of supported graph drivers to manpage
Add list of supported graph drivers to manpage

People looking to modify storage.conf might be confused as to
what the valid graph drivers and their names are. List all
supported drivers in the manpage to assist them.

Shouldn't be necessary elsewhere, most of the other fields are
just paths.

Signed-off-by: Matthew Heon <8dfcee637b682eb0c94f501440904ab12404ca8d@gmail.com>

rhatdan/storage,containers/storage,containers/storage,containers/storage,rhatdan/storage,rhatdan/storage