Allow any number of carbon-cache instances

Allow 0 or more carbon-cache instances on a host. Added the
gr_enable_carbon_cache flag. Also added the gr_cache_instances
parameter, which is a list of carbon-cache port configurations. Any
parameters for carbon-cache ports have been removed in favor of this
list of configurations.

Necessary changes to config files have been made, including additions to
the init script which will now correctly start/stop/restart all
carbon-cache instances on a machine.
Corrections to graphite-web configuration

Can now have graphite-web pointing to multiple carbon-cache instances on
the same host. Code for doing this existed before, but resulted in an
incorrect config file that caused graphite-web to fail to start.