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.

initializers/custom_fields.yml

@@ -1,3 +1,18 @@
+## Possible Choices:
+##   type:
+##   - text
+##   - integer
+##   - boolean
+##   - date
+##   - url
+##   - select
+##   filter_logic:
+##   - disabled
+##   - loose
+##   - exact
+##
+## Examples:
+
 # text_field:
 #   type: text
 #   label: Custom Text
@@ -22,8 +37,8 @@
 #   weight: 10
 #   on_objects:
 #   - tenancy.models.Tenant
-# selection_field:
-#   type: selection
+# select_field:
+#   type: select
 #   label: Choose between items
 #   required: false
 #   filter_logic: exact
@@ -41,8 +56,8 @@
 #     weight: 50
 #   - value: Fourth Item
 #     weight: 40
-# selection_field_auto_weight:
-#    type: selection
+# select_field_auto_weight:
+#    type: select
 #    label: Choose between items
 #    required: false
 #    filter_logic: loose