Create a method to seed basic models to bootstrap the system

Wee need to create a "Site" and a "User" associated with it in order to operate the system.
Historically this was done via TOSCA, but the new implementation use the gRPC APIs to communicate with the core, thus will require the user to be already present.

The plan to solve this is:

• move the migrations from xos-ui to xos-core (still using django for now: "python manage.py makemigrations")
• create a tempalte to generate a Django fixture in the cord-config step from platform-install (https://docs.djangoproject.com/en/1.11/howto/initial-data/)
• load the fixture before starting the core apis ("python manage.py loaddata <path-to-fixture>")
• start the core APIs

NOTE: assume someone has changed the admin user or site, and then the Core container is restarted. We want to make sure it doesn't revert the objects back to their original values. "loaddata" may not be the best way to achieve this, migrations can work better: https://docs.djangoproject.com/en/1.11/howto/initial-data/#providing-initial-data-with-migrations