Add celery worker to backend-app-1
Replace celery with path to celery in venv

This should allow starting celery without an active venv
Don't manage celery worker process in development
Refactor app resource class

This creates a new resource class, python_app, which the gunicorn_app
resource now uses.

This means that python_app can be used without implying a web app, e.g.
for a worker daemon.
Merge pull request #669 from alphagov/add-celery-worker

Add celery worker to backend-app
Declare virtualenv path in gunicorn_app
Merge pull request #672 from alphagov/add-celery-worker

Set virtualenv path in gunicorn_app
Upgrade sensu and module

Successfully provisioned a monitoring-1 and backup-box-1 with this,
after provisioning with master

Settings required for latest puppet module

 - Be explicit about the api
 - Need to specify a version of the sensu plugin gem
 - The puppet module changed the default vhost to not include a slash -
   https://github.com/sensu/sensu-puppet/pull/184

This unfortunately broke our setup, which expected a slash in the vhost.
Add uchiwa puppet module and config

Uchiwa replaces the sensu dashboard

Uchiwa defaults to using port 3000, so set the nginx alerts upstream to use that