use frame_id from cam info files, not camera name

current implementation uses camera name as frame_id but if we want to use stereo processing, both camera need to have same frame_id (for example, see 
http://answers.ros.org/question/10376/stereovslam-error-assertion-left_tfframe-right_tfframe/, or https://github.com/ros-perception/image_pipeline/blob/indigo/camera_calibration/nodes/cameracalibrator.py#L370 )
I'm not so strongly confident, but it seems using camera name in calibration file for frame_id is correct way.