Update controller_manager.cpp

getControllerNames now clears names before adding current names.  This fixes a bug in reloadControllerLibrariesSrv where the method is called twice in a row without first clearing the list.

Steps to reproduce:
- Spawn controller
- Stop controller
- reload-libraries

controller_manager.cpp:501: bool controller_manager::ControllerManager::reloadControllerLibrariesSrv(controller_manager_msgs::ReloadControllerLibraries::Request&, controller_manager_msgs::ReloadControllerLibraries::Response&): Assertion `controllers.empty()' failed.