dsbenchmark: configurable controller IP address

[coretutorials may want to have more Components in Bugzilla, this would be dsbenchmark component]

Hi Jan.

All planned system tests in releng silo
https://wiki.opendaylight.org/view/RelEng:Jenkins
will have this layout:

• Robot framework suite is placed on dynamic_robot.
• Controller(s) running on dynamic_controller(s).
• All other tools are placed on dynamic_mininet.

That means dsbenchmark.py will be running on different VM than controller,
so http://localhost:8181 will not reach ODL.
The python script needs to read IP address from command line, and use it instead of localhost.

If time permits, I will sumbit a patch to coretutorials.
This description is for the case when my time does not permit.