Make configuration access key fields editable.

Motivation / Problem:
Configuration settings are editable via the form at /admin/config/services/amazon/settings
If Access Key and/or Access Secret are filled in, then these fields are no longer editable. These fields should be editable.

Background:
The form generation script disables the input fields if there is data in them, but the intent was to determine if the keys were being managed in configuration management. This is not properly determining the management technique.

Proposed actions:
Make these fields editable until further work is completed.