[TEST] Replaced RestTestSuiteRunner with parametrized test that uses RandomizedRunner

ElasticsearchRestTests extends now ElasticsearchIntegrationTest and makes use of our ordinary test infrastructure, in particular all randomized aspects now come for free instead of having to maintain a separate (custom) tests runner

We previously parsed only the tests that needed to be run given the version of the cluster the tests are running against. This doesn't happen anymore as it didn't buy much and it would be harder to support as the tests get now parsed before the test cluster gets started. Thus all the tests are now parsed regardless of their skip sections, afterwards the ones that don't need to be run will be skipped through assume directives.

Fixed REST tests that rely on a specific number of shards as this change introduces also random number of shards and replicas (through randomIndexTemplate)