(#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.
Ok, looks like puppet has no ternary operator, it is the selector.  So I guess I am at loss as to what you would like me to do here.  Help?