Add "apparmor_parser" warning to check-config.sh

If AppArmor is enabled on the current system, but "apparmor_parser" isn't installed, it causes all kinds of issues.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #7157 from tianon/check-config-apparmor_parser

Add "apparmor_parser" warning to check-config.sh