[LOG4J2-787] Add lookup for JVM arguments. Better names and docs.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1619295 13f79535-47bb-0310-9956-ffa450edef68
[LOG4J2-787] Add lookup for JVM arguments. Add missing ctor.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1619302 13f79535-47bb-0310-9956-ffa450edef68