Add possibility to enable debug mode by setting environment variable in standalone scripts

It's common practice in Docker to control image run by configuring environment variables.

E.g.

docker run -e POSTGRES_PASSWORD=mysecretpassword -d postgres

The Cloud Enablement team adds such a possibility to EAP docker images. They replaces initial value of DEBUG_MODE:

# Enhance standalone.sh to make remote JAVA debugging possible by specifying
# DEBUG=true environment variable
sed -i 's|DEBUG_MODE=false|DEBUG_MODE="${DEBUG:-false}"|' $JBOSS_HOME/bin/standalone.sh

It's not a clean solution. This change should be made in original scripts and not on a Dockerfile level.

Please allow configuring the DEBUG_MODE from environment variables (in similar as e.g. the JAVA_OPTS are handled.