unlock the root account before running a sandbox

Fedora images come with their root account locked.
This prevents opening a ssh session.

controller/controller.py

@@ -723,6 +723,10 @@ export PATH="$PATH:/.toolbox/bin"
 touch       /root/.Xauthority
 chmod 600   /root/.Xauthority
 
+# unlock the root account in /etc/shadow
+# (needed at least on Fedora)
+usermod --unlock root
+
 exec /.toolbox/bin/sshd -D
             """.format(prepare=prepare)]