propellor/TODO

* Need a way to run an action when a property changes, but only
  run it once for the whole. For example, may want to restart apache,
  but only once despite many config changes being made to satisfy
  properties. onChange is a poor substitute.
* I often seem to want to be able to combine Properties monadically.
* --spin needs 4 ssh connections when bootstrapping a new host
  that does not have the git repo yet. Should be possible to get that
  down to 1.
* Make a way to express that a Property can be reverted (ie, installing a
  packages reverses to removing it). Then `reverted property` can be
  used to disable old properties.
* Display of docker container properties is a bit wonky. It always
  says they are unchanged even when they changed and triggered a
  reprovision.