ovn-nbctl: Help catch lport-set-addresses mistakes.

ovn-nbctl: Help catch lport-set-addresses mistakes.

While debugging a broken OVN environment yesterday, the problem turned
out to be invalid entries in the logical port addresses column.  In
particular, the following command had been used:

  $ ovn-nbctl lport-set-addresses lp0 MAC IP

instead of:

  $ ovn-nbctl lport-set-addresses lp0 "MAC IP"

This is really easy to mess up, so add some simple validation to the
lport-set-addresses command.  If the beginning of an argument is ever
an IP address, it's wrong.

In passing, also add a note to the ovn-nb db documentation to note that
the order of "MAC IP" is required, as "IP MAC" is not valid.

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