Reorder backend app role classes

Because alphabetz
Use 2 space indentation in all hiera yaml files

Because consistency (more files had 2 space indentation than 4)
Set up backdrop on dev role

- Add dependencies; clamav, google_credentials, mongo, python, varnish
- Add a dev varnish config and use it when in dev
- Add vhosts for backdrop and for varnish
Change development domain name

Adds a new environment file for the single node dev environment and
change the domain name for the multi-node environment to be inline with
preview, staging and production.
Merge pull request #166 from alphagov/setup-dev-vm

Set up backdrop on the dev role