Remove dummy os executor

[tomuben]

Background

Currently, as workaround, we have a DummyOsExecutor (in task TestRunnerDBTestTask) which is a fallback when the environment info is incomplete, which can happen if ITDE's SpawnTestEnvironment was launched for an external database. See exasol/integration-test-docker-environment#403
After the linked issue is fixed, we can remove the DummyOsExecutor and adapt the test DockerRunDBTestExternalDBTest, which then will need to manage the SSH file.

Acceptance Criteria

1. Use latest version of ITDE with fix for SpawnTestEnvironment for external database cannot be used with SSH executor integration-test-docker-environment#403
2. Remove the DummyOsExecutor in (in task TestRunnerDBTestTask)
3. Simplify TestRunnerDBTestTask._executor_factory()
4. Adapt the test DockerRunDBTestExternalDBTest
   • Use f"--db-os-access {DbOsAccess.SSH.name}" for run-db-test command
   • Manage the SSH Key file of the (external) docker db and add to run-db-test command the user and key-file