the bgp VM presents a router-on-a-stick.
For maximum flexibility you can configure bith the internal IP used for the endpoint as well as the peer interface information.  Here is the compete list of configuration required:

peer:
  AS
  link configuration (local IP, remote IP)

local:
  AS
  internal network (specify a host with netmask)

As an openstack VM the VM will run BGP over eth1 using the provided data in json format