Simplify nodes stats API

First, this breaks backwards compatibility!

* Removed /_cluster/nodes/stats endpoint
* Excpect the stats types not as parameters, but as part of the URL
* Returning all indices stats by default, returning all nodes stats by default
* Supporting groups & types in nodes stats now as well
* Updated documentation & tests accordingly
* Allow level parameter for "shards" and "indices" (cluster does not make sense here)

Closes #4057