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.
* Display of docker container properties is a bit wonky. It always
  says they are unchanged even when they changed and triggered a
  reprovision.
* Should properties be a tree rather than a list?
* Need a way for a dns server host to look at the properties of 
  the other hosts and generate a zone file. For example, mapping
  openid.kitenet.net to a CNAME to clam.kitenet.net, which is where
  the docker container for that service is located. Moving containers
  to a different host, or duplicating a container on multiple hosts
  would then update DNS too
* There is no way for a property of a docker container to require
  some property be met outside the container. For example, some servers
  need ntp installed for a good date source.
* Docking a container in a host should add to the host any cnames that
  are assigned to the container.
* Either `Ssh.hostKey` should set the sshPubKey attr
  (which seems hard, as attrs need to be able to be calculated without
  running any IO code, and here IO is needed along with decrypting the
  PrivData..), or the public key should not be stored in
  the PrivData, and instead configured using the attr.
  Getting the ssh host key into the attr will allow automatically
  exporting it via DNS (SSHFP record)
* Should be possible to generate a metapackage of all packages that
  properties direct apt to install. Then any other packages can be
  auto-removed. This would just be a matter of storing the apt-installed
  packages in an Attr. Although not removing essential and base packages
  could be problimatic.