Add label filter for docker system prune

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>
Upstream-commit: 41471dfe1c
Component: cli

components/cli/command/container/prune.go

@@ -49,7 +49,7 @@ const warning = `WARNING! This will remove all stopped containers.
 Are you sure you want to continue?`
 
 func runPrune(dockerCli *command.DockerCli, opts pruneOptions) (spaceReclaimed uint64, output string, err error) {
-	pruneFilters := opts.filter.Value()
+	pruneFilters := command.PruneFilters(dockerCli, opts.filter.Value())
 
 	if !opts.force && !command.PromptForConfirmation(dockerCli.In(), dockerCli.Out(), warning) {
 		return