Split the interface to change environments out from the minimal Backend
interface, to make it optional for backend implementations. If
backend.MultiState isn't implemented, return a "not implemented" from
environment related methods.
Have the Local backend delegate the MultiState methods to the proper
backend.
Backends are a mechanism that allow abstracting the behavior of
Terraform CLI from the actual core. This allows us to slip in special
behavior such as state loading, remote operations, etc.
James Bardin