ovn-nbctl: Update show format for addresses.

ovn-nbctl: Update show format for addresses.

This patch updates the formatting for the Logical_Port addresses column
in the show command output.  Previously, output would look like:

  addresses: 00:00:00:00:00:01 192.168.1.1 00:00:00:00:00:01 192.168.1.2

Now it looks like:

  addresses: ["00:00:00:00:00:01 192.168.1.1", "00:00:00:00:00:01 192.168.1.2"]

The grouping of addresses is important, so it should be reflected in the
output.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>