Improve service node launcher logic

[jihoonl]

171     def _start(self):
172 
173         launcher_type = self.msg.launcher_type
174 
175         if launcher_type == concert_msgs.ServiceProfile.TYPE_CUSTOM:
176             launcher = self.msg.launcher
177             launcher = launcher.split(" ")
178             self._proc = subprocess.Popen(launcher)  # perhaps needs env=os.environ as an argument
179         elif launcher_type == concert_msgs.ServiceProfile.TYPE_ROSLAUNCH:
180             self._start_roslaunch()
181         elif launcher_type == concert_msgs.ServiceProfile.TYPE_SHADOW:
182             pass  # no processes to start
183         else:
184             # Don't care - load_service_descriptions_from_service_lists will have validated the service description file.
185             pass

This is currently logic to start service node instance. Instead of method call like self._start_roslaunch(), we can do something like

FACTORY = {}
FACTORY['roslaunch'] = RoslaunchLauncher
FACTORY['shawdow'] = ShawdowLauncher
FACTORY['linkgraph'] = LinkGraphLauncher
FACTORY['blockly'] = BlocklyLauncher

self._launcher = FACTORY[SERVICE_TYPE]()
self._launcher.start()

[jihoonl]

Better way, dynamically load launcher class types instead hardcode in concert service manager.