Add optional private parameter config to Python server (ros-drivers#562)

* Made tcp_port a local parameter

The tcp_port parameter being a global parameter prevents the launch of multiple serial nodes on different port numbers using a launch file

* Added Fallback & Modified fork_server parameter

* Removed unnecessary defaults & added comments

rosserial_python/nodes/serial_node.py

@@ -54,9 +54,17 @@
     # TIOCM_DTR_str) line, which causes an IOError, when using simulated port
     fix_pyserial_for_test = rospy.get_param('~fix_pyserial_for_test', False)
 
-    # TODO: should these really be global?
-    tcp_portnum = int(rospy.get_param('/rosserial_embeddedlinux/tcp_port', '11411'))
-    fork_server = rospy.get_param('/rosserial_embeddedlinux/fork_server', False)
+    # Allows for assigning local parameters for tcp_port and fork_server with
+    # global parameters as fallback to prevent breaking changes 
+    if(rospy.has_param('~tcp_port')):
+        tcp_portnum = int(rospy.get_param('~tcp_port'))
+    else:
+        tcp_portnum = int(rospy.get_param('/rosserial_embeddedlinux/tcp_port', '11411'))
+
+    if(rospy.has_param('~fork_server')):
+        fork_server = rospy.get_param('~fork_server')
+    else:
+        fork_server = rospy.get_param('/rosserial_embeddedlinux/fork_server', False)
 
     # TODO: do we really want command line params in addition to parameter server params?
     sys.argv = rospy.myargv(argv=sys.argv)