Add sample playbooks to aid in conversion from lxb to ovn

These playbooks and the documented procedure provide one way of moving between LXB and OVN Neutron ML2 drivers. This considers the case of an OpenStack deployment using VXLAN tunnels and the Neutron L3 agent, moving to OVN with GENEVE tunnels.

This is intended as an example, as opposed to something which could be picked up and used with any OpenStack deployment, with no guarantees provided around functionality.

Users may wish to modify these playbooks to execute from an OSA deployment host as opposed to a generic host as used in this case.