(#MODULES-1332) set osfamily default for wsgi_socket_prefix

Add an osfamily based default class parameter value for the
wsgi_socket_prefix parameter of apache::mod::wsgi.  The default
value on RedHat osfamily platforms is '/var/run/wsgi'.  On other
platforms, use the Apache default.
Ah, ok.  Don't do it in the class initializor, just do it in the main body of the class.  Will that allow someone to override the value via hiera, or via a resource collector?  If so, then another patch will soon be forthcoming.