White list for capability server

I'd like to be able to specify the packages, which the capability server checks for existing interfaces and providers.

The reason being that in the future there will probably be a significant amount of interfaces. If one does not specify (default) providers for each existing interface, then warnings are thrown on capability server startup.

The rocon_gateway uses both a [black and white list](https://github.com/robotics-in-concert/rocon_multimaster/blob/hydro-devel/rocon_gateway/src/rocon_gateway/hub_manager.py#L31) and the app manager implements a white list by only loading apps, which are list in a file.

Example - app manager white list parameter (rapp_lists) set in the launch file:
https://github.com/robotics-in-concert/rocon_app_platform/blob/hydro-devel/rocon_app_manager/launch/standalone.launch