Fix issue when /etc/apparmor.d does not exists

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume@charmes.net> (github: creack)
Merge pull request #4624 from creack/fix_apparmor_init

Fix issue when /etc/apparmor.d does not exists
Merge branch 'master' of https://github.com/dotcloud/docker