Allow turning the security manager on on a per jvm basis

Turning on the security manager in a WildFly process is done by adding the -secmgr arg to org.jboss.modules.Main. But the jvm config settings in domain mode provide no mechanism for configuring args to Main; they only provide options for args to java. So there is no way to turn on the security manager for an individual jvm config (i.e. one or more servers) or if it is on for the HC there is no way to turn it off for a particular jvm config.

The jvm config needs an attribute for this; default is undefined which should be documented as meaning the setting is derived from the HC's settings (i.e. the way it is now.)