Major overhaul of graphite installation (removes Apache)

This PR does three things:

1. Makes the install directory of graphite configurable. No longer are you bound
   to installing graphite in /opt/graphite. This is exposed as the `$root_dir`
   parameter to `graphite`.

2. Completely decouples the installation of graphite from the system python
   site-packages, by installing graphite and all its dependencies into a
   virtualenv in the graphite root.

3. Uses gunicorn and upstart to run a graphite-web server on the specified port
   (`$port`) rather than setting up an Apache vhost with `mod_python`. The
   configuration of an nginx or Apache vhost is removed from this module.