containers: added mountns dir and mountns/mountns01.c

* tests shared mount: shared mount can be replicated to as many
  mountpoints and all the replicas continue to be exactly same.

Signed-off-by: Matus Marhefka <mmarhefk@redhat.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
containers: added mountns/mountns02.c

* Tests a private mount: private mount does not forward or receive
  propagation.

Signed-off-by: Matus Marhefka <mmarhefk@redhat.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>