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

++++++++ [Kindly note: This JIRA is raised to back-port the fixes made in JBPM-4518 to jBPM6.0.x branch ] ++++++++

• 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.