[storage] move choices to uncloud_storage

uncloud/uncloud_storage/models.py

@@ -1,3 +1,7 @@
 from django.db import models
+from django.utils.translation import gettext_lazy as _
 
-# Create your models here.
+
+class StorageClass(models.TextChoices):
+    HDD          = 'HDD', _('HDD')
+    SSD          = 'SSD', _('SSD')