Without much debugging it seems that there is no default set for "bgp_speaker_driver", because applying the patch yields the error shown in [1].
However setting

"bgp_speaker_driver = neutron_dynamic_routing.services.bgp.agent.driver.os_ken.driver.OsKenBgpDriver"

explicitly in "bgp_dragent.ini" seems to resolve the issue  ([2])

[1]
2019-08-07 15:24:06.302 6 INFO neutron.common.config [-] Logging enabled!
2019-08-07 15:24:06.303 6 INFO neutron.common.config [-] /var/lib/kolla/venv/bin/neutron-bgp-dragent version 14.0.2
2019-08-07 15:24:06.605 6 CRITICAL neutron [-] Unhandled error: AttributeError: 'NoneType' object has no attribute 'rpartition'
2019-08-07 15:24:06.605 6 ERROR neutron Traceback (most recent call last):
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/bin/neutron-bgp-dragent", line 10, in <module>
2019-08-07 15:24:06.605 6 ERROR neutron     sys.exit(main())
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron_dynamic_routing/cmd/eventlet/agents/bgp_dragent.py", line 20, in main
2019-08-07 15:24:06.605 6 ERROR neutron     bgp_dragent.main()
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron_dynamic_routing/services/bgp/agent/entry.py", line 45, in main
2019-08-07 15:24:06.605 6 ERROR neutron     manager='neutron_dynamic_routing.services.bgp.agent.bgp_dragent.'
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/service.py", line 414, in create
2019-08-07 15:24:06.605 6 ERROR neutron     periodic_fuzzy_delay=periodic_fuzzy_delay)
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/service.py", line 346, in __init__
2019-08-07 15:24:06.605 6 ERROR neutron     self.manager = manager_class(host=host, *args, **kwargs)
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron_dynamic_routing/services/bgp/agent/bgp_dragent.py", line 664, in __init__
2019-08-07 15:24:06.605 6 ERROR neutron     self).__init__(host, conf)
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron_dynamic_routing/services/bgp/agent/bgp_dragent.py", line 59, in __init__
2019-08-07 15:24:06.605 6 ERROR neutron     self.initialize_driver(conf)
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/osprofiler/profiler.py", line 160, in wrapper
2019-08-07 15:24:06.605 6 ERROR neutron     result = f(*args, **kwargs)
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron_dynamic_routing/services/bgp/agent/bgp_dragent.py", line 73, in initialize_driver
2019-08-07 15:24:06.605 6 ERROR neutron     self.conf))
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/importutils.py", line 44, in import_object
2019-08-07 15:24:06.605 6 ERROR neutron     return import_class(import_str)(*args, **kwargs)
2019-08-07 15:24:06.605 6 ERROR neutron   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/importutils.py", line 29, in import_class
2019-08-07 15:24:06.605 6 ERROR neutron     mod_str, _sep, class_str = import_str.rpartition('.')
2019-08-07 15:24:06.605 6 ERROR neutron AttributeError: 'NoneType' object has no attribute 'rpartition'
2019-08-07 15:24:06.605 6 ERROR neutron 


[2]
2019-08-07 15:25:08.683 6 INFO neutron.common.config [-] Logging enabled!
2019-08-07 15:25:08.683 6 INFO neutron.common.config [-] /var/lib/kolla/venv/bin/neutron-bgp-dragent version 14.0.2
2019-08-07 15:25:09.260 6 INFO neutron_dynamic_routing.services.bgp.agent.driver.os_ken.driver [-] Initializing os-ken driver for BGP functionality.
2019-08-07 15:25:09.260 6 INFO neutron_dynamic_routing.services.bgp.agent.driver.os_ken.driver [-] Initialized os-ken BGP Speaker driver interface with bgp_router_id=1.1.1.1
2019-08-07 15:25:09.310 6 INFO neutron_dynamic_routing.services.bgp.agent.bgp_dragent [-] BGP dynamic routing agent started
2019-08-07 15:25:09.335 6 INFO neutron_dynamic_routing.services.bgp.agent.bgp_dragent [req-8b3174d0-e5e5-40e8-8079-0a6a3f489933 - - - - -] Agent has just been revived. Scheduling full sync