Build and test RPMs for CentOS 7 & 8:
- Add a new RPM packaging workflow. We should probably merge this into the main packaging workflow.
- Move packaging scripts under pkg
- Move the generated Debian changelog under target/debian/.
- Inform the user what the `routinator-init` script is trying to do, useful in case it fails (e.g. because it can't find a way to become the 'routinator' user).
- Don't run routinator-init as sudo when root.
- Set cargo-deb search paths correctly for new pkg/ directory structure.
