Fixed notifyActions

By using isIdUsed method and removing slashes to fetch the observable models
Fix EBox::Model::Manager::_modelHasMultipleInstances

When the model is a children whose model parent is its grandfather or older
Notify when we are using a Domain in Dynamic DNS

When it is about to be removed
Merge pull request #1411 from Zentyal/ejhernandez/fix-notify-actions

Fix notifications on Dynamic DNS DHCP options