When plugins have a positive refcount, they were not allowed to be
removed. However, plugins could still be disabled when volumes
referenced it and containers using them were running.
This change fixes that by enforcing plugin refcount during disable.
A "force" disable option is also added to ignore reference refcounting.
Signed-off-by: Anusha Ragunathan <anusha@docker.com>
(cherry picked from commit 8cb2229cd1)
