Rename onie-stop to onie-discovery-stop

Per a good suggestion renamed to the more descriptive
onie-discovery-stop.  Also introduced the onie-discovery-start
command.

Testing:

Built images and exercised the new commands.

Reviewed By: Vidya Ravipati <vidya@cumulusnetworks.com>
Signed-off-by: Curt Brune <curt@cumulusnetworks.com>