Problem to start tomcat with a user whose group has a name different to the user

+++ This bug was initially created as a clone of Bug #1505762 +++
The tomcat systemd service unit has the Group option hardcoded to 'tomcat'. That is unnecessary and could cause issues when a different User is specified. We should remove the Group option and allow systemd to use the User option's default group to make it easier to use a user other than 'tomcat'. Removing the Group option also make the regular service unit function the same was as the JSVC service unit does.