Lazy load etree.

This import has been the bane of our existance.  If you call the
notifier too early this import will fail because it can't
find the shared libraries it needs.
We have worked around it time and time again, rejiggering startup
orders, manually setting LD_LIBRARY_PATH.

In this case it looks like the startup order is correct, yet it
still explodes with a traceback doing the import of etree that
isn't needed.

Ticket: #7118