Use the healthcheck middleware from oslo_middleware. This adds a new
pipeline that depends if /etc/magnum/healthcheck_disable file exists.
The healthcheck middleware is avalible under the /healthcheck URL.
Return values:
200 OK (If the file does not exist)
503 DISABLED BY FILE (If file exists)
Change-Id: I23179d5285831af12de7f392849c490d86a5682e
08d41a2de5