[TEST] Randomized number of shards used for indices created during tests

Introduced two levels of randomization for the number of shards (between 1 and 10) when running tests:

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

2) through `createIndex` and `prepareCreate` methods, 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 shards)

Added the following facilities to deal with the random number of shards:
- `getNumShards` to retrieve the number of shards of a given existing index, useful when doing comparisons based on the number of shards and we can avoid specifying a static number. The method returns an object containing the number of primaries, number of replicas and the total number of shards for the existing index

- added `assertFailures` that checks that a shard failure happened during a search request, either partial failure or total (all shards failed). Checks also the error code and the error message related to the failure. This is needed as without knowing the number of shards upfront, when simulating errors we can run into either partial (search returns partial results and failures) or total failures (search returns an error)

- added common methods similar to `indexSettings`, to be used in combination with `createIndex` and `prepareCreate` method and explicitly control the second level of randomization: `numberOfShards`, `minimumNumberOfShards` and `maximumNumberOfShards`. Added also `numberOfReplicas` despite the number of replicas is not randomized (default not specified but can be overwritten by tests)

Tests that specified the number of shards have been reviewed and the results follow:
- removed number_of_shards in node settings, ignored anyway as it would be overwritten by both mechanisms above
- remove specific number of shards when not needed
- removed manual shards randomization where present, replaced with ordinary one that's now available
- adapted tests that didn't need a specific number of shards to the new random behaviour
- fixed a couple of test bugs (e.g. 3 levels parent child test could only work on a single shard as the routing key used for grand-children wasn't correct)
- also done some cleanup, shared code through shard size facets and aggs tests and used common methods like `assertAcked`, `ensureGreen`, `refresh`, `flush` and `refreshAndFlush` where possible