ovn-nbctl: Update basic router commands.

ovn-nbctl: Update basic router commands.

A few minor changes related to router commands:

    - Use "lr" instead of "lrouter" to be more consistent with later
      changes.
    - Use the commands where possible in ovn unit tests.
    - Move documentation to group router commands together.
    - Update references from "lrouter" to "router".

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