QA: Decision about disk templates to use is more modular

qa_config.IsTemplateSupported() is already used to decide which disk
templates are supported by QA. Enabled templates now are part of that
decision.

Unit tests are updated.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>