Rework config keys

Improve the naming of the older config key.
We want to remove the unnecessary open_readspeaker prefixes to make the config keys more clear.

• open_readspeaker_i18n
• open_readspeaker_voice
• open_readspeaker_accountid
• open_readspeaker_url
• open_readspeaker_cdn_region

Proposed config keys

• lang
• voice
• customerid
• webreader_url
• cdn_region

lang, voice, customerid are used to make the query parameters in the current template.