Override bgp_pulse in test to be event-driven
Override bgp_pulse in test to be event-driven

bgp_pulse, in bgp_service, fires the sync method every constant amount
of time. In the tests, we want to verify this method is called. Since we
use loopingcall, which is tested, we can limit ourselves only to verify
the behaviour we want, and assume the method is called when we need it.

This change makes bgp_pulse event-driven, i.e. we can fire an event to
tell bgp_pulse to run, and therefore not have to wait for the timeout
(which doesn't always work and produces false positives).

Change-Id: I7f0bb3c89d6e1f198a8b9bd28e8c356033a234c6

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow