[TEST] Randomized number of shards used while testing

The default random number of shards is now set in the random index template, thus it affects all indices creations unless overwritten in the index creation within the specific test.

Some tests do require a specific number of shards, or a minimum number of shards, thus some common methods have been added to control that and be able to easily override it.

Adjusted also tests that relied on the default number of shards (5).