(#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.
I thought a selector would be more extensible (e.g. in case we needed to add another platform etc.), but I will change it.

I can't use an if/else here, can I?  That is, I need to use an expression, not a statement.  Do you mean a conditional e.g.

  $wsgi_socket_prefix = ($::osfamily == "RedHat") ? '/var/run/wsgi' : undef,

?