Allow virtualenv to use system wide packages

This fix the installation on some architecture by avoiding to compile lxml and also on architecture with low available ram (like the internet cube).

scripts/install

@@ -42,7 +42,7 @@ then
 	sudo mkdir -p $final_path
 fi
 sudo cp -r ../sources/* $final_path
-sudo virtualenv $final_path
+sudo virtualenv --system-site-packages $final_path
 sudo bash -c "source $final_path/bin/activate && pip install -r $final_path/requirements.txt"
 
 # Disable swapfile