Make OpenFaaS CE use the provider for load-balancing

This change removes the direct functions option which was
used originally for Docker Swarm. The Community Edition will
rely on the faas provider - faas-netes / faasd for load-balancing
of requests.

Direct Functions is required in order to delegate load-balancing
to Istio, Linkerd or some other kind of service mesh.

Tested by deploying a modified gateway image to a KinD cluster,
deploying the env function, and scaling to two replicas. This
balanced the load between the two pods by printing out the names
and then I ran a test with hey which returned 200s for all the
requests.

The prober which was part of the Istio support is no longer
required in the CE gateway so is removed for simplicity.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>