Add password-for-sudo to config

That way "sudo somecommand" will require entering the user's
password again (another password entry for consecutive sudo
requests won't be needed in the same session for the next
15min or so).

Since OctoPi has always shipped with allowing reboot and such
without sudo, even before Raspbian switched over to allowing
that for EVERY command, no further action is needed to keep the
preconfigured system commands working.

src/config

@@ -1,4 +1,4 @@
 export DIST_NAME=OctoPi
 export DIST_VERSION=0.15
-export MODULES="base(raspicam, network, disable-services(octopi))"
+export MODULES="base(raspicam, network, disable-services(octopi), password-for-sudo)"