command: show warnings from backend config validation

Previously we just silently ignored warnings from validating the backend
config, but now that we have a deprecated argument it's important to print
these out so users can respond to the deprecation warning.

command/meta_backend.go

@@ -1341,14 +1341,18 @@ func (m *Meta) backendInitFromConfig(c *config.Backend) (backend.Backend, error)
 
 	// Validate
 	warns, errs := b.Validate(config)
+	for _, warning := range warns {
+		// We just write warnings directly to the UI. This isn't great
+		// since we're a bit deep here to be pushing stuff out into the
+		// UI, but sufficient to let us print out deprecation warnings
+		// and the like.
+		m.Ui.Warn(warning)
+	}
 	if len(errs) > 0 {
 		return nil, fmt.Errorf(
 			"Error configuring the backend %q: %s",
 			c.Type, multierror.Append(nil, errs...))
 	}
-	if len(warns) > 0 {
-		// TODO: warnings are currently ignored
-	}
 
 	// Configure
 	if err := b.Configure(config); err != nil {