Add oslo config option for ip_prefix

Use "./bin/ryu-manager ryu/app/inception/inception.py" to run with the
default value

Or

Use "./bin/ryu-manager ryu/app/inception/inception.py --config-file
etc/ryu/inception.conf" to run with the value from the configuration
file
Merge pull request #3 from changbl/working

Add oslo config option for ip_prefix