Since we already know the name and namespace
of the service(s) we want we can just get the
correct one back from the API without filtering
the results.
* Potentialy saves a network hop
* Ability to configure LB algothim (given some work to expose an
anotation etc...)
* K8s config Watch is triggered far less often
If the flag kubernetes.namespaces is set...
Then we only select ingresses from that/those namespace(s)
This allows multiple instances of traefik to
independently load balance for each namespace.
This could be for logical or security reasons.
Addresses #336
