Disable by default plugins isolation
compare with .bytes() instead of ByteSizeValue.equals() in breaker service
[TESTS] Fix incorrect discovery options.
[DOCS] Added spring data elasticsearch integration
[DOCS] Various aggregation doc fixes
For unicast zen discovery don't overwrite a ping response for a node if the previous ping response has a set master and the current response hasn't.

Per single main ping request we maintain the received ping response per node. Each node level ping response is mapped into that. If from a previous node level ping request the response has already been set for a node, it will be overwritten. We give higher value to the latest response. This change makes sure that this doesn't happen if the previous response has a set master and the current response hasn't a set master. Otherwise a node will lose the fact that another node has elected itself as master, the result of that would be that there would multiple master nodes in a single cluster.

Closes #5413
[TESTS] Added AwaitsFix
Invoke postCollection on aggregation collectors.
Also cleanup how facet and aggs collector are used inside the QueryCollector

Closes #5387
[TEST] Randomized number of replicas used for indices created during tests

Introduced two levels of randomization for the number of replicas when running tests:

1) through the existing random index template, which now sets a random number of replicas that can either be 0 or 1 that is shared across all the indices created in the same test method unless overwritten

2)  through createIndex and prepareCreate methods, between 0 and the number of data nodes available, similar to what happens using the indexSettings method, which changes for every createIndex or prepareCreate unless overwritten (overwrites index template for what concerns the number of replicas)

Added the following facilities to deal with the random number of replicas:

- made it possible to retrieve how many data nodes are available in the `TestCluster`
- added common methods similar to indexSettings, to be used in combination with createIndex and prepareCreate method and explicitly control the second level of randomization: numberOfReplicas, minimumNumberOfReplicas and maximumNumberOfReplicas

Tests that specified the number of replicas have been reviewed:
- removed manual replicas randomization where present, replaced with ordinary one that's now available
- adapted tests that didn't need a specific number of replicas to the new random behaviour
- also done some more cleanup, used common methods like assertAcked, ensureGreen, refresh, flush and refreshAndFlush where possible