(#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.
Oh, woah, sorry. This is what I get for commenting with not enough coffee. I totally missed that this was happening in the class parameters...

So what should actually happen is a $wsgi_socket_prefix parameter should be added to `apache::params`, and then the default value should be set to `$::apache::params::wsgi_socket_prefix`.  Thanks!