Customfields from model template do not propagate to assets of that model

### Steps to reproduce
Set custom fields in a model configuration template Settings -> Asset Model
Add asset of that model to DataCenter -> Hardware
Open asset details
There are empty custom fields in Basic info of that asset details. I can find and add those ones (which were set in Asset Model) but I need to do it manually. Fields are not automatically populated from asset template.

### Expected behavior
Custom fields settings are populated to an asset of the particular model from Asset Model template.

### Actual behavior
Custom fields are not populating from Asset Model template

### Environment
* Ralph version: ii ralph-core 20191001.1 amd64 Ralph is an DCIM/CMDB - asset man
* Operating system: Operating System: Ubuntu 18.04 LTS, Kernel: Linux 4.15.0-22-generic
* Method of installation: Deb package? Manual installation? Docker?
virtual machine in cloud
installation as directed on https://ralph-ng.readthedocs.io/en/stable/installation/installation/