allow a container to run in a new cgroup namespace.
When running in a new cgroup namespace, the current cgroup appears to
be the root, so that there is no way for the container to access
cgroups outside of its own subtree.
By default it uses --cgroup=host to keep the previous behavior.
To create a new namespace, --cgroup=private must be provided.
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>