SSH key generation was split out into its own systemd service in
https://github.com/NixOS/nixpkgs/pull/372979, but dependent service
definitions weren't updated.
The `apply-ec2-data` service needs to run before SSH key generation,
as it fetches host keys defined in ec2 user data and these keys should
take priority over generating new ones. Currently, the ordering
doesn't specify which should run first of `apply-ec2-data` and
`sshd-keygen`; in practice it seems that `sshd-keygen` often wins the
race, though.
Update the dependencies so that `apply-ec2-data` always runs first.
d9ac3ba30b