Update and improve bagpipe driver documentation
Update and improve bagpipe driver documentation

Update to:
- explain how to enable the OVS agent extension
- remove "enable_plugin bagpipe-bgp"

More precision and detail.

Change-Id: I69652dff0fe2788f1e6ce92e55ca50c3c8481ebe

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