Add possibility to disable transaction management in DBIntegrationSpec and AppIntegrationSpec

[eugeneRover]

DBIntegrationSpec has no possibility to disable transaction management for individual tests.
The only thing that affects the transaction management is rollback configuration option. Unfortunately the option is static therefore it changes behavior for all tests, which is definitely unacceptable (IMO it's mostly unusable).

It would be great if DBIntegrationSpec (and AppIntegrationSpec) had mentioned possibility, just like DBSpec has.

[ipolevoy]

@eugeneRover , this is complete. See the docs: https://javalite.io/testing#database-transaction-management