Add function to load YAML files ...

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

Update Custom Fields Initializer for Netbox 2.7 ...

The custom field 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 custom_fields.yml file as
follows:

- type must be lowercase
  - the `selection` type was changed to `select`
- the filter_logic must be lower case

This is to achieve compatibility with the naming schema that
Netbox uses internally. It allows us to become forward compatible
in case Netbox ever introduces a new type for custom fields.

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

Ignore missing initializers ...

Previously, the startup_scripts would fail if an initializer file was
not present. Now they just ignore missing files.

Increase order prefix to 3 digits