* Services that need password files will automatically depend on the
appropriate NixOps key service as necessary.
* New `sourcedFile` option for Rails applications to load a Bash
script just before starting the Rails service. Useful for setting
secret environment variables.
This is necessary for files like the state file that tells the
migration script whether or not the schema needs to be loaded.
Fixes a bug where the migration script would try to load the schema on
a second deployment (since touching a file in the db directory fails
because it's in the nix store).
