Fixes # [JBPM-4519] Back-port the changes made in [JBPM-4518] to jBPM6.0.x branch

This Pull Request has been sent to back-port the code changes made to "JBossUserGroupCallbackImpl" from master branch to jBPM6.0.x branch.
This code enhancement made to the "JBossUserGroupCallbackImpl" code would enable it to support the "org.jbpm.ht.user.separator" property and would customize this UserGroupCallback implementation's current ability for separating Group Ids from a Property .