ovn-nbctl: Update logical switch commands.

ovn-nbctl: Update logical switch commands.

    A few minor changes related to logical switch commands:

        - Use "ls" instead of "lswitch" to be more consistent with other
          command changes.
        - Use commands where possible in ovn unit tests.
        - Update references from "lswitch" to "ls" (code) or "switch" (user).

Signed-off-by: Justin Pettit <jpettit@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>
Acked-by: Ben Pfaff <blp@ovn.org>