QA: Refactored launching of the various instance tests

The code launching instance tests for "plain" and "drbd" templates has been
unified. In this way it's easier to add more templates, and to re-use the same
tests with different configurations.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>