software/rapid-cdn: make re6st-verification-url optional
While working on deploying a frontend without Internet access, I noticed that a promise, re6st-connectivity.py, was failing every time. This promise tries to access to the URL http://[2001:67c:1254:4::1]/index.html and fails if it cannot. I am unsure of why this promise existed at the first place, but it seems that we might want to keep it to check for Internet connection on the frontend.
What this MR aims at doing is therefore to make the parameter optional: it will default to nothing, and the promise section will not be generated in instance.cfg when the parameter is unset, which by default will make the frontend work even without Internet access, or access to a filtered Internet, while also allowing to check for network connectivity if needed.
I am quite unsure if I did things right for this MR, so feel free to comment about anything. Tests are running, so we will see if it breaks anything.