Fix acrarollback tests

[Lagovas]

1. Found, that tests for acra-rollback do nothing and don't check that last query returns anything before iteration:

result = self.engine_raw.execute(acrarollback_output_table.select())
        result = result.fetchall()
        for data in result:
            self.assertIn(data[0], source_data)

And they succeed even with errors like key 'keypair1' not found.
So, fixed the name of keypairs and added additional check that there are anything to iterate.

2. Found, that our GetDriverConnectionStringHost method incorrectly parses usernames that don't look like valid schema for net.URL. So, replaced custom parsing logic to use an appropriate function from the target driver that knows how to deal with its own custom connection string.

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation in the Acra documentation site or has PR on documentation repository
  with new changes
• CHANGELOG.md is updated (in case of notable or breaking changes)
• CHANGELOG_DEV.md is updated
• Benchmark results are attached (if applicable)
• Example projects and code samples are up-to-date (in case of API changes)