motd - update-motd.d file must be executable

the files created by this in /etc/update-motd.d must
be executable scripts for this to work.

per default it will create them as 0644 and run-parts
will not execute it, so the output won't end up in /run/motd

Change-Id: I3e3a6f7408bd8ced944aaea50d274bada0aa7d40