Add configurable workspace prefix for S3 Backend

[rowleyaj]

Fixes #13184, addresses part of #15358

@jbardin @apparentlymart this is my first foray into the world of Go so go please go easy :)

Does this address the points raised in the other issue? The tests still pass although I'm not sure how to add some specifically related to this. I've also tested it locally and it successfully migrated my dummy setup from the old state structure to the new.

$ tree
.
├── env: # Old structure
│   ├── testing
│   │   └── v1
│   │       └── terraform-remote-state-example
│   ├── testing2
│       └── v1
│           └── terraform-remote-state-example
├── terraform-state # New Structure
│   ├── testing
│   │   └── v1
│   │       └── terraform-remote-state-example
│   └── testing2
│       └── v1
│           └── terraform-remote-state-example
└── v1 # Default workspace
    └── terraform-remote-state-example

terraform {
  required_version = "~> 0.10"

  backend "s3" {
    bucket  = "rowleyaj-tf-state-demo"
    key     = "v1/terraform-remote-state-example"
    region  = "us-east-1"
    encrypt = true

    dynamodb_table = "rowleyaj-terraform-state-lock"
    workspace_key_prefix = "terraform-state"
  }
}

[rowleyaj]

Not sure why tests are failing on this but I don't think it's related, any help appreciated:

--- FAIL: TestResourceProvider_stop (2.05s)
	resource_provisioner_test.go:80: should finish
FAIL
FAIL	github.com/hashicorp/terraform/builtin/provisioners/local-exec	4.192s

[rowleyaj]

Tests pass now, was a strange timeout :)

[josephholsten]

Looks good, but we may want to mention the default in the doc

[josephholsten]

should we mention the default is "env:"?

[jbardin]

Thanks!
Just a minor doc-string update before merging, but otherwise, LGTM!

[jbardin]

Can we mention that it's for non-default workspaces here? Maybe:

applied to the non-default state path

[ghost]

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