Add support for custom Graphite Web port

This changeset allows the default Apache VirtualHost port to be
customized via `graphite_web_port`.
Make Graphite component installs idempotent

Installations with `python setup.py install` are not idempotent by
default. This changeset adds a `creates` argument to the `command`
directive that executes `python setup.py install` so that it gets
skipped if an installation already occurred.