Add 'unix:' prefix in the gunicorn configuration

When you're configure a webapp::python::instance and leave the 'socket'
parameter by default you end up with a unusable configuration, because
the gunicorn bind socket config[0] isn't prefixed with "unix", but the
nginx configuration it is.

This patch makes this parameter consisten bewteen nginx and gunicorn
configurations.

[0] http://docs.gunicorn.org/en/latest/settings.html#bind