apparmor: add mkdir/rmdir interface to manage policy namespaces

When setting up namespaces for containers its easier for them to use
an fs interface to create the namespace for the containers
policy. Allow mkdir/rmdir under the policy/namespaces/ dir to be used
to create and remove namespaces.

BugLink: http://bugs.launchpad.net/bugs/1611078

Signed-off-by: John Johansen <john.johansen@canonical.com>