1156 - fix multiselect filters in agent view
There were two problems:
- ng-table now expect a promise instead of a deferred object for filter options
- angularjs-dropdown-multiselect now requires a valid object to track but unfortunately, filters are dynamic in ng-table so I created a wrapper just to ensure the model for the current filter is always defined
Closes #1156 - ccmanager error when using table dropdown filter in agent view