Fixes https://github.com/openfaas/faas/issues/687

 allowing the
gateway to handle the responsibility of basic auth for when it is
in use.

To enable set basic_auth env-var to true and then mount two
secrets or plaintext files under /var/secrets/

basic_auth_user, basic_auth_password

Tested with faas-cli list/deploy and with Safari browser.

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