Improved README-bagpipe for devstack use
Improved README-bagpipe for devstack use

More information:
- what enable_service statement to use
- how to run fakerr

Change-Id: Id4fd4bb678ca1686ede1af8ab942ad9e920bbf0c

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn