Tweaks to Elasticsearch tasks

#### 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.