add-user.sh doesn't correctly handle property files where final line does not terminate with a new line

As an example when adding an application user with the add-user.sh script the role(s) are not added correctly as they are added to the end of an existing (commented out line).

For example after adding user admin with roles admin standalone/configuration/application-roles.properties contains:

...
#guest=guestadmin=admin

which results in the role not being available, this should obviously have been added to a new line like so:

...
#guest=guest
admin=admin