better test bucket management

[ppolewicz]

We need to create a pytest factory which will produce fixtures (create buckets on demand), and once the test is over, the factory will delete all created buckets. Currently we pass fixture bucket_name to all tests, but 1) it creates only one bucket, while there should be many sometimes 2) it doesn't allow pass parameters to bucket creation, but in some tests we need non-default bucket configuration during creation. When passing bucket_factory to each test, the test will call bucket_factory(params), and this will create bucket and auto-clean after test.