ovn-nbctl: Add lrouter and lrport related commands.

ovn-nbctl: Add lrouter and lrport related commands.

ovn-nbctl provides a shortcut to perform commands related lswitch, lport
and such but it doesn't have similar commands related to logical routers
and logical router ports. Also, 'ovn-nbctl show' is supposed to show an
overview of database contents, which means it should show the routers
as well. "ovn-nbctl show LSWITCH" shows the switch details, similarly
"ovn-nbctl show LROUTER" should show the router details too. This patch
takes care of all of these.

Modifications;
1) ovn-nbctl show -- will now show lrouters as well
2) ovn-nbctl show <lrouter> -- will show the router now

New commands added:
3) ovn-nbctl lrouter-add [LROUTER]
4) ovn-nbctl lrouter-del LROUTER
5) ovn-nbctl lrouter-list
6) lrport-add LROUTER LRPORT
7) lrport-del LRPORT
8) lrport-list LROUTER
9) lrport-set-mac-address LRPORT [ADDRESS]
10) lrport-get-mac-address LRPORT
11) lrport-set-enabled LRPORT STATE
12) lrport-get-enabled LRPORT

Unit test cases have been added to test all of these modifications and
additions.

Signed-off-by: Nirapada Ghosh <nghosh@us.ibm.com>
[blp@ovn.org added features to match the lswitch and lport commands]
Co-authored-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>