Create VM for fpcentral

We need a new VM to run fpcentral, a panopticlic-like website that we want to use to check that the Tor Browser fingerprint does not change.

As this is not a big application, I think we don't need a lot of CPU, memory, or disk space.

We can run this on Debian stable, and we need the git, python3 and python3-virtualenv packages installed.

An hostname pointing to this VM would also be useful, such as fpcentral.tbb.torproject.org.