install Python 3 packages in a virtualenv

The cryptography package has created all sorts of installation trouble over the last few years, probably because of mismatches between OS-installed packages and pip-installed packages. Using a virtualenv for all Python packages used by the management daemon should make sure everything is consistent.

See #1298, see #1264.

management/daemon.py

@@ -1,5 +1,3 @@
-#!/usr/bin/python3
-
 import os, os.path, re, json, time
 import subprocess