Add cgroup delete opt for ns deletion

Closes #3305

This adds an opt and a `--cgroup,-c` flag to `ctr namespaces rm` to
remove the cgroup that is commonly created with runtimes.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/ctr/commands/namespaces/namespaces_linux.go

@@ -0,0 +1,31 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package namespaces
+
+import (
+	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/namespaces"
+	"github.com/urfave/cli"
+)
+
+func deleteOpts(context *cli.Context) []namespaces.DeleteOpts {
+	var opts []namespaces.DeleteOpts
+	if context.Bool("cgroup") {
+		opts = append(opts, containerd.WithNamespaceCgroupDeletion)
+	}
+	return opts
+}