Make 'prog' overridable

The prog name for jboss is 'hard coded' to be 'jboss-as', however when integrating JBoss on its system
one my prefer to use a custom script and override the name (to name 'jboss-eap' for instance or use
the name of the application, when it is only running one application). The variable 'prog' has been
therefore renamed PROG and has become overridable, like the other JBOSS_...
Rename PROG variable to JBOSS_PROG_NAME:

As PROG is a pretty generic name, and as it can now be overriden from outside
the script, it seems appropriate to prefix it with JBOSS_, like the other overrideable
names in the script, but also append _NAME to make the variable name more meaningful