Add an environment variable to select individual gateways for testing purposes
To all effects, location is the atomic gateway selection for users. We fetch info from menshen, and then we randomize the individual gateways if they're in the same location (this needs to change when we have load info: it should be randomize same load then).
But for testing, we need to select individual gateways (by FQDN). Do this as a env var shortcut.
It was argued in the past that for testing we can use other scripts, but in this case what I'm interested in testing belongs to the pkg/vpn/bonafide packages, and the whole (at times complex) interaction with the firewall, openvpn/openssl version etc... so I'd rather test in with the same production clients.