doc: update host dependency section

Change python to python3 and mention the new "python3-distutils" that we
will need in buster or Ubuntu 18.

Closes: #41
Signed-off-by: Henning Schild <henning.schild@siemens.com>

doc/user_manual.md

@@ -58,7 +58,8 @@ git
 debootstrap/jessie-backports# for debian >= "buster"
 dpkg-dev
 parted
-python
+python3
+python3-distutils           # in case your host is host > debian 9
 qemu
 qemu-user-static
 rxvt-unicode                # build_parallel