Correct docs for carbon_(source|content)

The docs refer to `carbon_conf_(source|content)` but the params are called
`carbon_(source|content)`. I prefer the former name, but that would break
backwards compat, so standardise on the latter.
Refactor spec tests for ::config

Move two config related spec tests from `graphite_spec.rb` to
`graphite__config_spec.rb`, with other config related items.

Refactored that target spec to fit them into the correct context.
Refactor spec tests for ::install

De-dupe tests for Graphite and Carbon installs. Test that Whisper is
installed with the correct version.
Refactor spec tests for ::service

Move them to a separate spec file. Per the other sub-classes.
Optional support for carbon-aggregator

Allow carbon-aggregator[0] to be configured and enabled. This is used by
GOV.UK to calculate deploys and HTTP status code rates. Class docs should be
fairly self explanatory.

[0]: https://graphite.readthedocs.org/en/0.9.12/carbon-daemons.html#carbon-aggregator-py
Force carbon-aggregator to run in foreground

This is the same dirty hack applied to carbon-cache in #13. Otherwise the
following happens, and variations of `expect` cause it to hang :(

```
root@graphite-1:~# start carbon-aggregator
carbon-aggregator start/running, process 23813
root@graphite-1:~# status carbon-aggregator
carbon-aggregator stop/waiting
root@graphite-1:~# cat /var/log/upstart/carbon-aggregator.log
Starting carbon-aggregator (instance a)
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
```
Merge pull request #16 from gds-operations/carbon_aggregator

Optional support for carbon-aggregator