Decouple Nginx configs from graphite::web & relegate to subclass

It looks like we'll need to use Apache to serve Graphite, because we need LDAP
authentication and we don't build Nginx with the LDAP module. So make
graphite::web agnostic about web servers. It simply sets up uWSGI to serve
graphite at unix:/var/run/graphite-web/graphite-web.sock. Automatic Nginx
configuration is still available via graphite::web::nginx.

Change-Id: I9925474134371d390c88f1f3164c6803e3f73dba
Merge "Decouple Nginx configs from graphite::web & relegate to subclass" into production