[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