Enable authentication for the traffic generators

Now that we have fixed a bug that allowed for unauthorized access to some Stork views we need to modify the DHCP and DNS traffic generators to create the session (login) prior to getting configured DHCP subnets from the server. Without this, the traffic generators fail t get the list of subnets and this makes them unusable.

The useful reference I got from @godfryd was: https://requests.readthedocs.io/en/master/user/advanced/