Handle primary IPs on devices and VMs

Add optional primary_ip on virtual machines initializer

Add function to load YAML files ...

This commit starts to remove some code redundancy from the startup
scripts for easier maintenance.

Update VM Initializer for Netbox 2.7 ...

The vm database model has changed in Netbox 2.7.
Therefore the initializer code, that was used before, broke.
As a user, you will need to update your virtual_machines.yml
file as follows:

- Make sure the status is spelled lowercase.

See the diff of this commit for further information how this is meant.

Added newline and breaks.

Fixed further requirements.

Fixed aesthetics.