vagrant: use debian/ubuntu pip/virtualenv packages

this is cleaner than easy_install3 / installing virtualenv with pip.

Vagrantfile

@@ -31,8 +31,14 @@ def packages_debianoid(user)
     # this way it works on older dists (like ubuntu 12.04) also:
     # for python 3.2 on ubuntu 12.04 we need pip<8 and virtualenv<14 as
     # newer versions are not compatible with py 3.2 any more.
-    easy_install3 -i https://pypi.python.org/simple/ 'pip<8.0'
-    pip3 install 'virtualenv<14.0'
+    if which easy_install3 2> /dev/null; then
+        # works up to xenial / stretch
+        easy_install3 -i https://pypi.python.org/simple/ 'pip<8.0'
+        pip3 install 'virtualenv<14.0'
+    else
+        # works on recent debian / ubuntu
+        apt-get install -y python3-pip virtualenv python3-virtualenv
+    fi
   EOF
 end