QA: Added exclusive_storage to qa_config

QA configuration now tracks the state of the exclusive_storage flag at
cluster level. This will be used to selectively enable tests according to
the configuration.

Also, it's now possible to specify the initial value of the
exclusive_storage flag in the configuration file.

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