#5330 (fix) Fee override gets erased at hummingbot start

https://github.com/hummingbot/hummingbot/pull/5330

When conf_fee_overrides_TEMPLATE.yml does not contain the same list of exchanges supported by hummingbot, load_yml_into_cm mechanic does not update the ConfigVar with the values from the user's conf_fee_overrides.yml and the save ends-up erasing the value set by the user in his file The quick fix is to populate TEMPLATE with the correct list. The rest of the files are to add test methods