[OVN] IPI install fails on MTU 1242

Description of problem:

From OCPBUGS-2921.

OCP used to work in an OpenStack cluster with MTU 1242.

MTU 1242 is less that 1280 so IPv6 will not function. NetworkManager seems to change the bootstrap node MTU from 1242 to 1280 unless IPv6 is disabled. Bootstrap node doesn't seem to be able to disable IPv6 so it has an MTU mismatch (1280 != 1242) and install fails

Version-Release number of selected component (if applicable):

4.13.0-0.nightly-2023-03-07-081835

How reproducible:

Always

Steps to Reproduce:

1. Create a DHCP environment with MTU 1242
2. Create MachineConfig that disables IPv6 for all roles
3. IPI baremetal install
4. Check bootstrap MTU. 

Actual results:

bootstrap node MTU is 1280 and IPv6 is not disabled.

install fails

Expected results:

IPI baremetal install succeeds with MTU 1242 on all nodes.

Additional info:

The OCP minimum MTU requirements are not documented anywhere?

MachineConfig trying to disable IPv6 on bootstrap.

---
apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  creationTimestamp: null
  labels:
    machineconfiguration.openshift.io/role: bootstrap
  name: 11-bootstrap-disable-ipv6
spec:
  kernelArguments: ["rd.debug", "ipv6.disable=1"]