Fix parser for ovn-nbctl show command output
Fix parser for ovn-nbctl show command output

Since openvswitch v2.6.0 output format of `ovn-nbctl show` has
changed. Any occurrences of "lswitch" have been replaced with "switch",
and occurrences of "lport" have replaced with just "port". Adapt to the
changes and drop support for OVN < 2.6.0.

openvswitch/ovn-scale-test,openvswitch/ovn-scale-test