Debian stock kernel has CONFIG_CGROUP_MEM_RES_CTLR disabled due to the
overhead [1]. Allow to start containers if the corresponding files in
the cgroup filesystem are missing. This fixes Debian bug #566180 [2].

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=534964
[2] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=566180