[TEST] Ensure cluster size reflected in the cluster state

We perform some management operations that require the cluster to be
consistent with respect to the number of nodes in the cluster state
/ visible to the master in order to rely on the ack mechanism. This
only applies to the test infrastructure when nodes are not explicitly
started / stopped as well as while tearing down the cluster and wiping
indices after the tests.
[TEST] size should start from 1, top_hits aggregation doesn't support size <= 0
[TOOLS] Added simple S3 Download tool based on boto