This introduces a new document called `EncryptionPolicy` to configure
this behavior. It currently only supports using symmetric encryption
with `GPG`, but that should be available on all Ubuntu systems (which is
what we currently support) and should also be fairly reliable.
Change-Id: I06d4faa119b736773df0d8cbf0e7a23fd98edcdf
Depends-On: https://review.openstack.org/#/c/602175/