pass the port for the dagroot as argument on the command line

When starting openvisualizer, it may be helpful to specify a port to be configured as dagroot (e.g. for experiments automation)

Example:
sudo scons runweb --dagroot-port=/dev/ttyUSB0

NB: currently, a single port is accepted, but I may modify the program to accept several ports, such as:
sudo scons runweb --dagroot-port=/dev/ttyUSB0,/dev/ttyUSB1