Add a way to check the status of the services.

(rather than only displaying their status)

- Add a connection string to the services. A connection-string is in the form
  {protocol}://{hostname}:{port}.
- Add a celery task able to check the status of a service, given a protocol.
- Add a "check_status" django command.
- Update the base settings to use celery
- use the settings/deploy.py if it exists
- explicitely pin the versions in the requirements.txt file
- display the connection string on the detail page of the service
merge ametaireau/status-check with master