Fix test suite's field "slapos parameters" line length
The my_cluster_configuration field was transformed into a proxy field.
The target is a new field with id my_view_mode_json_text_area (following the base as email and phone fields). It includes JSON validator and the error message for invalid JSON from old my_cluster_configuration field.
Waiting for test results in Nexedi's ERP5.
@seb: since you suggested the proxy field, can you review, please?