improve error message for common elasticsearch discovery problems

guess default HTTP endpoint for elasticsearch and try to read config

* compare cluster.name
* check compatible version
* warn if no connection could be made

fixes #506

(cherry picked from commit 2db6a92a51b8efd620fbc26879f69dc2e6e40bce)

Conflicts:
	graylog2-server/src/main/java/org/graylog2/bindings/providers/IndexerProvider.java
	graylog2-server/src/main/java/org/graylog2/indexer/Indexer.java
	graylog2-server/src/test/java/org/graylog2/indexer/IndexerTest.java