Allow user to run web app under mod_wsgi and allow user to specify a custom port for Apache vhost

This pull requests adds two features:

* Allow user to specify a custom port for the Apache vhost (`node[:graphite][:graphite_web][:port]` attribute)
* Add support for running webapp using mod_wsgi (`node[:graphite][:graphite_web][:use_mod_wsgi]` attribute)

People have been discouraged from using mod_python for years now (project is dead, bad performance, multiple bugs and issues, etc.), but this cookbook still uses it.

I added a new attribute which allows you to use this cookbook with mod_wsgi. Because mod_python is dead we should eventually fully remove support for mod_python. For now I kept the support there so people don't have problems with migrations.

* http://blog.dscpl.com.au/2010/06/modpython-project-is-now-officially.html