We previously used a less-than-optimal way of storing the configuration
values for Graphite/Carbon, which meant you had to override the entire
configuration (including restating default values, etc) to change one
thing.

This now mirrors how we approach things in the Grafana role, where each
distinct value is a distinct variable that can be overridden separately,
and is pulled together at a higher level into the configuration.

This playbook will install carbon-cache and graphite-api to provide a
minimalistic starting point for metrics collection.

It currently installs all components from their upstream source (albeit
with defaults that point to tags, and not master/development branches).
It uses systemd, and so is tailored specifically to Ubuntu 16.04.

It currently does not support any of the advanced configuration such as
multiple carbon-cache process + carbon-relay.  It is also lacking many
variables to fully tweak the various configuration files, although
there's support for fully configuring the storage schemas.

There is a small "master" playbook -- edx-monitoring -- for running the
role, and in the future, supporting the culmination of other playbooks
to build out an all-in-one monitoring server: Graphite, Grafana, etc.