ovn-ctl: Add commands to manage OVN DB ovsdb-servers individually

ovn-ctl: Add commands to manage OVN DB ovsdb-servers individually

This patch adds the following functions
  - start_nb_ovsdb, stop_nb_ovsdb, restart_nb_ovsdb to start, stop and
    restart the OVN NB DB ovsdb-server independently.
  - start_sb_ovsdb, stop_sb_ovsdb, restart_sb_ovsdb to start, stop and
    restart the OVN SB DB ovsdb-server independently.

These commands can be used to run ovsdb-server for each DB in a separate
container.

Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Andy Zhou <azhou@ovn.org>