This fix tries to address the issue raised in 29999 where it was not
possible to mask these items (like important non-removable stuff)
from `docker system prune`.
This fix adds `label` and `label!` field for `--filter` in `system prune`,
so that it is possible to selectively prune items like:
```
$ docker container prune --filter label=foo
$ docker container prune --filter label!=bar
```
Additional unit tests and integration tests have been added.
This fix fixes 29999.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
41471dfe1c