admin.sh doesn't define mode in else case

The else case (no ADMIN env variable set) depends on mode variable that has not been defined: https://github.com/wildfly/wildfly-cekit-modules/blob/master/jboss/container/wildfly/launch/admin/added/launch/admin.sh#L51

The issue is hidden in EAP/XP/CD context because some other scripts define mode that leaks into this script. The problem can be seen in WildFly s2i. Nevertheless we should fix it in all contexts.

The leak seems caused by https://github.com/wildfly/wildfly-cekit-modules/blob/master/jboss/container/wildfly/launch-config/os/added/launch/launch-common.sh#L48 that, as a side effect, makes the variable global.