WFLY-1604 - Embed the chosen realm name within a comment in the properties file.

This eliminates a long standing issue with add-user where by users were asked for the name of the realm for each new user added even though different realms for different users was never an option.

This is also a pre-requisite change to allowing users to specify a custom name for their properties file, in that case we no longer know if this is for management for applications or for something else.  

This will also enable the implementation of another feature to allow every WildFly installation to have it's own custom realm name, this would mean that pre-prepared hashed from one installation would not be usable against another installation even if the same username and password was used as the realm name would make the hashes unique.