runtests: pass remaining arguments as apps to the runner.

And remove the '-a' option. It was pretty custom, and allowed only
a single application to be passed. Now you can pass multiple apps,
in the same form as for the runner.
Add tests for SSL auth.
SSLCertAuthBackend: make request mandatory.

django.contrib.auth is prepared to call backends which don't have
matching prototype. Therefore, with mandatory 'request' the backend
simply won't be called for non-SSL requests.