Update request_specs when instance's spec changing

Currently, when creating an instance, the instance information is recorded in the request_specs of the nova-api db, such as the flavor attribute, the available_zone, etc., and this information is not changed as the attribute of the instance itself changes. For example, after the migrate instance, or evacuate instance, the available_zone of the instance may be changed. Or after the resize, the flavor attribute of the instance will be changed, but these changes of the instance will not update the request_specs table.

Therefore, when the current spec property of the instance has changed, and the request_specs table is not updated, the rebuild/resize/(from shelved to unshelve)/evacuate operation is performed on the instance. When these operators are need to obtain the instance spec from the request_specs table (The spec record which may not match the spec of the current instance) to do some config for the instance, which may cause the instance to have an uncontrollable exception while performing the related operation.

So, it is necessary to update the request_specs table in time when the instance spec changes.