QA: Test enabled disk templates with known configuration

The test failed because it was executed also in a configuration that wasn't
expected, i.e., with exclusive storage enabled.

Also the docstring is shortened, as it was being truncated in the QA output.

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