Fixes # [JBPM-4518] JBossUserGroupCallbackImpl should support "org.jbpm.ht.user.separator" property to customize "Group Id" separator for groups

This Pull Request has been sent to enhance the "JBossUserGroupCallbackImpl" so that it supports "org.jbpm.ht.user.separator" property to customize this UserGroupCallback implementation's current ability for separating Group Ids from a Property .
Previously it was only separating multiple Group Id's by "," separated values. But with this current code changes it should be able to give the users the ability to customize this separator and it would allow users to use character like "," in it's group name.