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>

namespaces/store.go

@@ -33,5 +33,14 @@ type Store interface {
 	List(ctx context.Context) ([]string, error)
 
 	// Delete removes the namespace. The namespace must be empty to be deleted.
-	Delete(ctx context.Context, namespace string) error
+	Delete(ctx context.Context, namespace string, opts ...DeleteOpts) error
 }
+
+// DeleteInfo specifies information for the deletion of a namespace
+type DeleteInfo struct {
+	// Name of the namespace
+	Name string
+}
+
+// DeleteOpts allows the caller to set options for namespace deletion
+type DeleteOpts func(context.Context, *DeleteInfo) error