Add "onie-" prefix to various ONIE CLI commands

The important ONIE commands "installing a NOS" and "install a ONIE
self update" were a little hard to find.  Formally the commands were:

install_url -- install a NOS using a URL
update_url  -- install a ONIE self update using a URL

These commands are renamed:

  install_url --> onie-nos-install
  update_url  --> onie-self-update

Added symlinks from the old names to the new names to maintain
backwards compatibility.

A new command is added:

  onie-stop   --> stop the discovery process

Testing:

Built images and exercised the new and old commands.

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