Introduce a new list of controllers just containing "all".

Make the lists of controllers null-terminated.

If the cgmanager api version is high enough, use the 'all' controller
rather than walking all controllers, which should greatly reduce the
amount of dbus overhead.  This will be especially important for
those going through a cgproxy.

Also remove the call to cleanup cgroups when a cgroup existed.  That
usually fails (and failure is ignored) since the to-be-cleaned-up
cgroup is busy, but we shouldn't even be trying.  Note this can
create for extra un-cleanedup cgroups, however it's better than us
accidentally removing a cgroup that someone else had created and was
about to use.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>