Refactor DataprocCreateBatchOperator and Dataproc system tests

[moiseenkov]

Refactored DataprocCreateBatchOperator:

• significantly refactored the execute() method for decreasing its accumulated complexity and code duplication.
• made the batch_id parameter optional as it is supported by API
• made the region parameter required because (1) it is required by the API, and (2) it was already required de-facto because the operator used to raise and exception manually: raise AirflowException("Region should be set here")
• added a specific error message to the operator logs (in both deferrable=True|False modes), so it would be more convenient for users to debug their batch jobs using the operator logs directly.

Also additionally slight refactored Dataproc system tests:

• reduced parallelism
• added retry to cluster creation tasks in a hope to suppress the error

This PR also rolls back changes in pre-commit hook made for Dataflow system tests. From now those changes rea not needed.