This fix adds `--filter enabled=true` to `docker plugin ls`, as was specified in 28624. The related API and docs has been updated. An integration test has been added. This fix fixes 28624. Signed-off-by: Yong Tang <yong.tang.github@outlook.com> Upstream-commit: a66e0dc349dad2d45685846a95fe1d6da967a46d Component: engine