macaroons: add SetRootKey method

This adds a SetRootKey method which can be used to set the default
macaroon root key to a specified value.

macaroons/service.go

@@ -68,6 +68,10 @@ type ExtendedRootKeyStore interface {
 	// GenerateNewRootKey calls the underlying root key store's
 	// GenerateNewRootKey and returns the result.
 	GenerateNewRootKey() error
+
+	// SetRootKey calls the underlying root key store's SetRootKey and
+	// returns the result.
+	SetRootKey(rootKey []byte) error
 }
 
 // Service encapsulates bakery.Bakery and adds a Close() method that zeroes the
@@ -300,6 +304,16 @@ func (svc *Service) GenerateNewRootKey() error {
 	return nil
 }
 
+// SetRootKey calls the underlying root key store's SetRootKey and returns the
+// result.
+func (svc *Service) SetRootKey(rootKey []byte) error {
+	if boltRKS, ok := svc.rks.(ExtendedRootKeyStore); ok {
+		return boltRKS.SetRootKey(rootKey)
+	}
+
+	return nil
+}
+
 // ChangePassword calls the underlying root key store's ChangePassword and
 // returns the result.
 func (svc *Service) ChangePassword(oldPw, newPw []byte) error {