Add function to load YAML files ...

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

Update Interfaces Initializer for Netbox 2.7 ...

The interface 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 dcim_interfaces.yml
file as follows:

- Make sure the type is a value out of the possible choices.

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

Fixing typos.

Fixed further requirements.

Fixed aesthetics.