Rename remote_state plugin to backend

[tleguijt]

Rename plugin to backend since it avoids confusion with the MACH composer plugins, and is more in line with how terraform names it.

We might also consider renaming the values itself;

• aws -> s3
• gcp -> gcs

[demeyerthom]

We might also want to change remote_state to state as we also support local as an option. So then you get

terraform_config:
  state:
    backend: s3

Obviously we want this to be backwards compatible, so that means a lot of checks ;)