(#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.
Merge pull request #852 from richm/bug/modules-1332

(#MODULES-1332) set osfamily default for wsgi_socket_prefix