qa_config: Remove exclusive storage flag from config

Commit 6a0f22e1 added the ability to track the status of the cluster-wide
flag for exclusive storage. It did so in a way which modifies the
in-memory configuration dictionary. This patch changes the code to use a
plain global variable as a temporary solution until a configuration
object is added.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>