Fix description of elasticsearch.cluster_health

Looks like this was copy/pasta from the task above it.
Pretty format Elasticsearch cluster_health output

So that the JSON output is more human readable.
Add elasticsearch.cluster_nodes task

This is good for diagnosing split brain where each node has a different view
of the cluster.

I'd like to replace a lot of the instructions in the opsmanual that
reference the head plugin with these commands, because they're safer.
Merge pull request #85 from alphagov/es_tweaks

Tweaks to Elasticsearch tasks