Split out the backend environment interface

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.

backend/backend.go

@@ -27,13 +27,22 @@ type Backend interface {
 	// to load the state. If the state.State is a state.Locker, it's up to the
 	// caller to call Lock and Unlock as needed.
 	State() (state.State, error)
+}
 
+// MultiState is an interface that a backend can implement to allow changing
+// between named states depending on the configured environment.
+type MultiState interface {
 	// States returns a list of configured named states and the current state.
 	States() ([]string, string, error)
 
-	// ChangeState changes to the named state. If this doesn't exist it'll be
+	// ChangeState changes to the named state. If the named state doesn't exist
-	// created.
+	// it will be created.
 	ChangeState(name string) error
+
+	// DeleteState removes the named state if it exists. If the current state is
+	// deleted, the backend should change to the default state. It is an error
+	// to delete the default state.
+	DeleteState(name string) error
 }
 
 // Enhanced implements additional behavior on top of a normal backend.

backend/local/backend.go

@@ -27,6 +27,8 @@ const (
 	DefaultBackupExtension = ".backup"
 )
 
+var ErrEnvNotSupported = errors.New("environments not supported")
+
 // Local is an implementation of EnhancedBackend that performs all operations
 // locally. This is the "default" backend and implements normal Terraform
 // behavior as it is well known.
@@ -117,6 +119,15 @@ func (b *Local) Configure(c *terraform.ResourceConfig) error {
 }
 
 func (b *Local) States() ([]string, string, error) {
+	// If we have a backend handling state, defer to that.
+	if b.Backend != nil {
+		if b, ok := b.Backend.(backend.MultiState); ok {
+			return b.States()
+		} else {
+			return nil, "", ErrEnvNotSupported
+		}
+	}
+
 	// the listing always start with "default"
 	envs := []string{backend.DefaultStateName}
 
@@ -154,6 +165,15 @@ func (b *Local) States() ([]string, string, error) {
 // DeleteState removes a named state.
 // The "default" state cannot be removed.
 func (b *Local) DeleteState(name string) error {
+	// If we have a backend handling state, defer to that.
+	if b.Backend != nil {
+		if b, ok := b.Backend.(backend.MultiState); ok {
+			return b.DeleteState(name)
+		} else {
+			return ErrEnvNotSupported
+		}
+	}
+
 	if name == "" {
 		return errors.New("empty state name")
 	}
@@ -174,6 +194,15 @@ func (b *Local) DeleteState(name string) error {
 
 // Change to the named state, creating it if it doesn't exist.
 func (b *Local) ChangeState(name string) error {
+	// If we have a backend handling state, defer to that.
+	if b.Backend != nil {
+		if b, ok := b.Backend.(backend.MultiState); ok {
+			return b.ChangeState(name)
+		} else {
+			return ErrEnvNotSupported
+		}
+	}
+
 	name = strings.TrimSpace(name)
 	if name == "" {
 		return errors.New("state name cannot be empty")