Make log config from rosgraph optional

[otamachan]

Hi,

I'm trying to run rospy in a pure Python environment.
Currently rospy.init_node() always fails because rospkg.RosPack().get_path('rosgraph') fails in a pure Python environment(see below).

This PR enables that init_node does not fail even when rosgraph cannot be found by rospkg.

Thank you.

>>> rospy.init_node('test')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/otamachan/proj/rospy/venv/lib/python3.6/site-packages/rospy/client.py", line 309, in init_node
    rospy.core.configure_logging(resolved_node_name)
  File "/home/otamachan/proj/rospy/venv/lib/python3.6/site-packages/rospy/core.py", line 405, in configure_logging
    _log_filename = rosgraph.roslogging.configure_logging('rospy', level, filename=filename)
  File "/home/otamachan/proj/rospy/venv/lib/python3.6/site-packages/rosgraph/roslogging.py", line 156, in configure_logging
    rosgraph_d = rospkg.RosPack().get_path('rosgraph')
  File "/home/otamachan/proj/rospy/venv/lib/python3.6/site-packages/rospkg/rospack.py", line 203, in get_path
    raise ResourceNotFound(name, ros_paths=self._ros_paths)
rospkg.common.ResourceNotFound: rosgraph

[dirk-thomas]

Thanks for the patch.