Migrated Sensu Configuration to Leverage Graphite Checks.

To ensure metrics and alerts are always reporting the same values** it has been necessary to update the sensu logic to replace the majority of the checks with ones that talk back to graphite to get the data. NB: At present these tests are run on the actual servers but this may change over time.

As part of this change a number of smaller changes occurred as defined in the CHANGELOG

* Moved sensu community plugins so they are only available from /etc/sensu/community
* Added custom graphite plugin based on original community plugin
* Modified most of the existing checks to use graphite metrics to ensure alerts and
  metrics are drawing data from the same source
* Added client checks for cron and collectd




** based on the standard monitoring mantra "We alert on what we graph!"