Enable healthcheck middleware

It would be valuable to configure the healthcheck middleware [0] for all services deployed by fuel, available since Kilo.

Several (obvious) benefits:
- Provide a common API for healthcheck across OpenStack services
- HAproxy performs more accurate HTTP checks for its backend status
- Operators can disable a service (maintenance mode, test, ..) by creating a simple file w/o stopping the service.
- monitoring systems can leverage this API to provide insight information of service status with a lighten checks w/o authentication (in opposition to heavily check for specific resources)

fuel-specs is coming in the next future, here some implementation details:

* Configure WSGI pipelines for each project "api-paste.ini"
* For security reason as mentioned in the original spec [1], the 'healthcheck' URI mustn't be open to the WWW:
   * restricted access to local/management network or by auth
   * or random/configurable URI at deployment time
* HAproxy "httpchk" option will be "GET /<healthcheck path>" instead of the default "OPTION /"

[0] http://docs.openstack.org/developer/oslo.middleware/healthcheck_plugins.html
[1] http://git.openstack.org/cgit/openstack/oslo-specs/plain/specs/kilo/oslo-middleware-healthcheck.rst