fix(scripts/motdgen): use a timer instead of polling dbus

motdgen can take a ridiculous amount of CPU when you have lots of units.
I talked with David Strauss and he wants similar functionality and is
working with upstream to make it cheap to do. Until then run this on a
timer.
Merge pull request #94 from philips/motdgen-timer

fix(scripts/motdgen): use a timer instead of polling dbus