Ordering of DCIM models break dumpdata/loaddata functionality

[jeremystretch]

Environment

• Python version: 3.6.8
• NetBox version: 2.6.11

Steps to Reproduce

1. Create at least one cable terminated to a power feed
2. Export existing data with ./manage.py dumpdata -o <filename> --natural-foreign --natural-primary --indent 4 --format json
3. Load the JSON-formatted data into a new database with ./manage.py loaddata <filename>

Expected Behavior

All data should be imported successfully

Observed Behavior

An AttrbiuteError is raised:

AttributeError: Problem installing fixture '/home/jstretch/netbox/netbox/../demo_data.json': 'NoneType' object has no attribute 'cable'

This is because Django is exporting Cables prior to exporting PowerFeeds. Upon import, Django is attempting to create cables to terminations that don't yet exist, hence the exception.

Presumably, this can be fixed by moving the Cable model definition so that it comes after PowerFeed in models.py.