Description of problem:
- With the current code of "JBossUserGroupCallbackImpl" (including the code in master) it is restricting users from using special characters like "," in their "Group Id" names. 
e.g. if we use the following Group Id to set for a User in a Property , the "JBossUserGroupCallbackImpl" treats "maryg" and "g" as two Group Id in place of treating "maryg,g" as a single Group Id.
~~~
...
        Properties properties = new Properties();
        properties.setProperty("krisv", "krisvgg");
        properties.setProperty("mary", "maryg,g");
        properties.setProperty("john", "johngg");      
    
        UserGroupCallback userGroupCallback = new JBossUserGroupCallbackImpl(properties);
...
~~~

It is because of this restriction in it's code which only checks for "," comma for separating multiple Group Ids when a user enters it's group names inside a Property.
~~~
			groups = Arrays.asList(userGroups.getProperty(userId, "").split(","));
...
~~~

- Please enhance the "JBossUserGroupCallbackImpl" code so that it allows the usage of "org.jbpm.ht.user.separator" property to customize "Group Id" separator for groups.

Version-Release number of selected component (if applicable):
- jBPM 6.0.3-redhat-6, 6.0.3-redhat-7 (i.e. including Roll Up patch#1 for BPMS 6.0.3)

How reproducible:
- Always

Steps to Reproduce:
1. Import the attached "jBPM6_CustomSeparatorBugTest.zip" project in JBDS/Eclipse IDE.
2. Follow the instructions provided in the README.md file inside the project which tells you how to add a group with "Group Id" (containing a character ",") to a user and try to claim and complete a task.
3.

Actual results:
- User Task does not get listed for the specified user "mary" as if "mary" is not the potential owner for the User Task.

Expected results:
- Ideally "JBossUserGroupCallbackImpl" should have recognized "maryg,g" as a complete Group Id.

Additional info:
- Please refer to "https://issues.jboss.org/browse/JBPM-4518" which is the upstream JIRA raised with the pull request.