Enhancement: Added isMember() method to modResource
Added additional method listAssignedGroups()/getAssignedGroupsList()
Cleanup / Added method getResourceGroupNames() instead of earlier added methods
Refactored joinGroup()/leaveGroup() methods to make us of isMember()
Added modResource.isMember and modResource.getResourceGroupNames methods

* origin/pr/11364:
  Refactored joinGroup()/leaveGroup() methods to make us of isMember()
  Cleanup / Added method getResourceGroupNames() instead of earlier added methods
  Added additional method listAssignedGroups()/getAssignedGroupsList()
  Enhancement: Added isMember() method to modResource