Include the realm name in the users.properties file

Continually prompting for the realm name within add-user.sh has caused problems as it gives the impression that this can be specified for each user added - however all the users within the file need to have the same realm otherwise the pre-prepared hashes will not be valid.

If there is no realm contained within a specially formatted comment then add-user.sh will prompt for the realm name as normal and will write the realm name to the file - if the comment is present add-user.sh will just use it without prompting.

If multiple properties files are found for adding a user and the realms do not match the update will fail with an appropriate error message.