Switch to external auth

This commit moves the OpenFaaS gateway from using in-process
basic-auth for everything to use an external auth URL instead.

When auth is not enable, this functionality is not added to the
handlers and behaves as before. When enabled, the configured
plugin with authenticate requests.

Tested on Docker Swarm with positive and negative tests.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>