This makes set up easier for new users. Users can still set
FISSION_NAMESPACE but it will default to "fission". Users can also
still set KUBECONFIG, but it will default to $HOME/.kube/config.
Also, update the post-install chart notes.txt and the install guide to
use "fission function test" as the first step after install. This
means that if there's anything wrong with the setup, the user will see
useful errors instead of "internal server error". Also, they can test
their setup without worrying about nodeports or ingresses or whatever.
All existing functionality of FISSION_URL and FISSION_ROUTER continues
to work.
