[JENKINS-47043] Introduce exit lifecycle to exit instead of restart

If hudson.lifecycle.ExitLifecycle is selected, jenkins will exit when
restart is requested. Exit code is taken out of
jenkins.model.Jenkins.exitCodeOnRestart, default is 5.

This is usable when there is an external watchdog such as SystemD.

SystemD service example:

   ExecStart=@/usr/bin/java jenkins $JAVA_OPTIONS -DJENKINS_HOME=${JENKINS_HOME} -Dhudson.lifecycle=hudson.lifecycle.ExitLifecycle -Djenkins.model.Jenkins.exitCodeOnRestart=55 -jar $JENKINS_WAR --debug=${JENKINS_DEBUG_LEVEL} --httpPort=${JENKINS_PORT} --httpListenAddress=${JENKINS_LISTEN_ADDRESS} $JENKINS_ARGS

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>