Add a way to check the status of the services.

[almet]

(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

[bfirsh]

Thanks! I've left a few comments on the commit.

[almet]

Thanks for taking the time to review that, I don't think I'll be able to find the time to actually commit myself to tidy up everything, so I'll let whoever wants to do it proceed.

(this code was made something like a year ago, so there are possible flaws in there, yup).

[bfirsh]

No worries! Good to get this lined up for PR. I may have time to fix it myself at some point, but I'm quite busy at the moment.

[bsdlp]

thanks @ametaireau, I'll try to work through some of @bfirsh's suggestions and get it ready for merge