Update Custom Field logic for Netbox v2.10.x

Add function to load YAML files ...

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

Update VLAN Initializer for Netbox 2.7 ...

The VLAN 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 vlans.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.