Copy a repo

[luckydonald]

Is your feature request related to a problem? Please describe.
I have source1 and want to copy to backend1

Describe the solution you'd like
With #145 we have a copy feature.

location:
  from: source1
  to:
    - backend1 # fast
    - backend2 # slow
  copy:
    backend1:
      - backend3
      - backend4

however, how do I copy it ONLY from source1 to backend1?

I want to basically run

export RESTIC_REST_USERNAME="..." 
export RESTIC_REST_PASSWORD="..." 
restic -r ./repo copy --from-repo rest:https://${RESTIC_HOST}