Make recurring period a database model

- For easier handling (foreignkeys, many2many)
- For higher flexibility (users can define their own periods)

uncloud_vm/serializers.py

@@ -101,8 +101,8 @@ class VMProductSerializer(serializers.ModelSerializer):
         read_only_fields = ['order', 'owner', 'status']
 
 class OrderVMProductSerializer(VMProductSerializer):
-    recurring_period = serializers.ChoiceField(
-            choices=VMWithOSProduct.allowed_recurring_periods())
+    # recurring_period = serializers.ChoiceField(
+    #         choices=VMWithOSProduct.allowed_recurring_periods())
 
     def __init__(self, *args, **kwargs):
         super(VMProductSerializer, self).__init__(*args, **kwargs)
@@ -133,8 +133,8 @@ class DCLVMProductSerializer(serializers.HyperlinkedModelSerializer):
     """
 
     # Custom field used at creation (= ordering) only.
-    recurring_period = serializers.ChoiceField(
-        choices=VMProduct.allowed_recurring_periods())
+    # recurring_period = serializers.ChoiceField(
+    #     choices=VMProduct.allowed_recurring_periods())
 
     os_disk_uuid = serializers.UUIDField()
     # os_disk_size =