Use OpenShift 4.6 ignition string for static IPs, if VCD ever supports vm_advanced_config parameters

[tseelbach]

This issue serves 2 purposes:

1. track the possibility that we can setup static IPs for OpenShift nodes thru the ignition string if VCD vm_advanced_config parameters will be supported by VCD

2. Describe the change (workaround?) we made from our 4.5 ignition in vcd_toolkit to what we did to support OCP 4.6 in terraform-openshift4-vcd.

The new capability in OpenShift 4.6 to pass Static IP's thru the ignition string (guestinfo.afterburn.initrd.network-kargs="ip=::::::none nameserver=srv1 [nameserver=srv2 [nameserver=srv3 [...]]]") does not work in the VCD environment because the ignition strings depend on VMWare vm_advanced_config parameters, which aren't available in VCD. RedHat Bugzilla 1913791 has been opened, but its really a VCD issue.

To get around this issue, we have come up with a scheme for DHCP reservations for the OpenShift servers based on MAC address. In terraform-openshift-vcd there is now a DHCP Server that runs on the LoadBalancer VM which performs this function. This means that you cannot use DHCP in the Edge Gateway for the subnet you are using for OCP. If you start DHCP services in the Edge Gateway, make sure you don't include this subnet.