Execute lxc default_password_scrub when needed

default_password_scrub run every time chef runs, which is annoying since
we report our resource updates to a campfire room. This stops that from
happening on every run.

This was commented out in https://github.com/hw-cookbooks/lxc/commit/32558eb4f5195960ece726415294c895b6a446cd, but I'm not 100% sure why.