backend: Operation.Environment renamed to "Workspace"

This is part of an effort to switch this terminology across all of
Terraform.

backend/backend.go

@@ -136,8 +136,9 @@ type Operation struct {
 	// The duration to retry obtaining a State lock.
 	StateLockTimeout time.Duration
 
-	// Environment is the named state that should be loaded from the Backend.
-	Environment string
+	// Workspace is the name of the workspace that this operation should run
+	// in, which controls which named state is used.
+	Workspace string
 }
 
 // RunningOperation is the result of starting an operation.

backend/local/backend_local.go

@@ -23,7 +23,7 @@ func (b *Local) Context(op *backend.Operation) (*terraform.Context, state.State,
 
 func (b *Local) context(op *backend.Operation) (*terraform.Context, state.State, error) {
 	// Get the state.
-	s, err := b.State(op.Environment)
+	s, err := b.State(op.Workspace)
 	if err != nil {
 		return nil, nil, errwrap.Wrapf("Error loading state: {{err}}", err)
 	}

command/meta_backend.go

@@ -168,7 +168,7 @@ func (m *Meta) Operation() *backend.Operation {
 		PlanOutBackend:   m.backendState,
 		Targets:          m.targets,
 		UIIn:             m.UIInput(),
-		Environment:      m.Workspace(),
+		Workspace:        m.Workspace(),
 		LockState:        m.stateLock,
 		StateLockTimeout: m.stateLockTimeout,
 	}