qa_instance: refactoring wrt disk templates

This patch is a preparation for later patches in QA in this series
it refactors the instance QA to make it more flexible regarding
creation of instances with different disk templates. Right now we
only support creation of instances with disk templates 'drbd', 'plain',
and 'diskless'. With our current plans to improve storage handling,
we should make QA more flexible to also create instances of other
templates. This patch restructures the code in a way that instances
can be created more easily to be used in other QA scripts.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtarara@google.com>