Fix several reasons for flakes in external gateway e2e tests

* Add node as neighbor to external gateways to avoid timeouts and speed
  up tests. Cluster nodes don't know where to send ARP replies to
  requests from the fake IPs that are added to the containers so force
  an entry on the neighbor table with a ping. Otherwise it takes time
  for the containers to learn the node address risking timeouts.
* Fixes test timeouts which where based on the number of ping
  attempts rather than actual time.
* Add a timeout to BFP pairing check.

Signed-off-by: Jaime Caamaño Ruiz <[email protected]>

Author: jcaamano