Update neutron-with-existing-external-network

[elad-eyal]

Comments for https://www.rdoproject.org/networking/neutron-with-existing-external-network/ - I was deploying on CentOs 8

1. I had a problem with assigning the user a role in the project (after "openstack user create"). I ended up adding the user all the different roles via web gui.

2. Instead of the instructions on setting fixed IP I followed the video at the end of this page which shows how to make the bridge use DHCP.

3. Huh I didn't get that private_network is not the same as the external_network. Maybe add a note or use completely different subnet (not 100 and 122)

4. I kept seeing the message "No enough DHCP Agents" is neutron log - but that is a false alarm, a DHCP Agent is not needed.

5. It kept "unable to bind". I had to do this:

facter fqdn 
yielded ==> packstac
sudo ovs-vsctl get Open_vSwitch . external-ids:hostname 
yiedled ==> localhost
sudo ovs-vsctl set Open_vSwitch . external-ids:hostname="packstack"

6. I had a problem where it would not download the correct cirros image. So I wanted to edit the answer file myself. So I took the command packstack --allinone --provision-demo=n --os-neutron-ovs-bridge-mappings=extnet:br-ex --os-neutron-ovs-bridge-interfaces=br-ex:eth0 --os-neutron-ml2-type-drivers=vxlan,flat and added --gen-answer-file answer.txt and went ahead to edit it. It seems to me that the argument --os-neutron-ovs-bridge-interface was supposed to edit CONFIG_NEUTRON_OVN_BRIDGE_IFACES in the answer file but it did not. So I edited CONFIG_NEUTRON_OVN_BRIDGE_IFACES accordingly.
   (So this may be a problem in code, not in the document)

Thanks @karelyatin for lots of help!!

[karelyatin]

os-neutron-ovs-bridge-interface configures CONFIG_NEUTRON_OVS_BRIDGE_IFACES only, for configuring CONFIG_NEUTRON_OVN_BRIDGE_IFACES, os-neutron-ovn-bridge-interface needs to be used. #1388 fixes other issues. This issue can be closed.