core/network.py: Silence scapy functions

And also, _scapy_srp  is only meant to be a wrapper. It's out of its scope
to set specific values to parameters, save for the verbose one, because
we don't want to have annoying messages in the stdout.
network.ManInTheMiddle: Decrease packets interval to 0.2

I've tried to attack a Sony Xperia Z1, running Android 4.3, and an interval
of 0.5 seconds doesn't have the desired effect. After decreasing it to 0.2,
the attack goes well on the device.