"healthcheck --collect" command discovers healthchecks

[benoitbryon]

As platform maintainer, when I run Django project's healthcheck --collect command, I get a list of project's healthchecks, i.e. those to be executed via healthcheck command.

Here is a proposal:

• iterate over installed apps. Use registry of applications: https://docs.djangoproject.com/en/1.8/ref/applications/
• for each application, following hospital's convention, look for "healthchecks" modules or packages
• by default, ignore "tests" modules and packages
• the output can be parsed, i.e. list of healthchecks can be extracted from healthcheck --collect output