[OVN][Doc] DevStack guide: Add command to get the  port's UUID
[OVN][Doc] DevStack guide: Add command to get the  port's UUID

It just makes it simpler for the user to copy & paste a command to
obtain the port's UUID.

Change-Id: Ib839c8ed1e78f14d49690367a68f007a68c5cebe
Signed-off-by: Lucas Alvares Gomes <ef89b923b1145c9ec2fd96eee513279016a7a963@gmail.com>

mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron