Docs: Adding community-contributed Scout plugins

Scout plugins for reporting key node, cluster, and index health metrics.

Closes #7937
Docs: Fix order for PUT _mapping docs

Closes #8083
Docs: Added RethinkDB River to list of community plugins

Closes #8016
Docs: Update query-string-query.asciidoc

Update sample

Closes #8024
Docs: fix typo

Closes #8111
Docs: Typo: s/by/be/

Closes #8114
Improve amibiguous ES_HEAP_SIZE warning in deb init script

Closes #3554
More improvements to debug logging for upgrade test.
Remove unnecessary log level setting in upgrade test.
Fixing copy/paste mistake in SearchRequest.extraSource's exception message.
Docs: Documented the http.*host and transport.*host settings
Docs: Update id-field.asciidoc

It is strange to provide an example with `"store" : false` when talking about possibility of enabling the field to be stored.
Broke the line in the mapping in two lines for better readability.
More verbose sentence above the mapping.

Closes #7894
Docs: Some Data Cant't be obtained.

In previous case, the first 100 data can't be obtained.

Closes #8103
Docs: Percolate `_score` reference

Added missing `_score` word, made the sentence less ambiguous.

Closes #8115
Docs: Percolate reference - a typo and a misused word

Closes #8116
Avoid shading of org.joda.convert package, fixes #3557
Aggregations: Buckets can now be serialized outside of an Aggregation

This change means that buckets can now be serialised to JSON and serialized and deserialized to the transport API outside of the aggregation that contains them.  This is a required change for #8110 (Reducers framework) but should make sense on it's own since object should really take care of their own serialization rather than relying on their parent object.