Install apparmor tools also for Ubuntu Focal.

k8s gate is still on focal, so patch which unblock the apparmor for
jammy does not affect it. Here is the fix for focal as well.

Change-Id: I2a9bc69a59e7d6d21d61e79115d5a3c726c73ab0
(cherry picked from commit bdc0b49ce3)

devstack/lib/docker

@@ -59,6 +59,7 @@ function install_docker {
     local arch
     arch=$(dpkg --print-architecture)
     if is_ubuntu; then
+        apt_get install apparmor
         if [[ ${dist_version} == 'trusty' ]]; then
             if uname -r | grep -q -- '-generic' && dpkg -l 'linux-image-*-generic' | grep -qE '^ii|^hi' 2>/dev/null; then
                 apt_get install linux-image-extra-$(uname -r) linux-image-extra-virtual