Make sensu check independent.

There is no reason why the sensu check definition should be defined
on the elasticsearch node. This only works right now because we typically
only use elasticsearch on the monitoring server.

Make it a regular sensu check, with a subscriber list.

In the process:

* Make the warning and critical thresholds for heap usage configurable.
* Make the 'graphite  node name' overridable (* is only fine if you have one ES node)
* Up the default warning and critical thresholds - 70% usage is *fine* for heap, and
  we're getting lots of false alarms.
* Make dependent on sensu-formula v4.4.0, as we are now not quoting the target name
update CHANGELOG
Merge branch 'master' into fix_up_monitor
Merge pull request #15 from ministryofjustice/fix_up_monitor

Make sensu check independent.