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