Change data type of all TLS cert and key params back to String

[lbetz]

refs #623

[zilchms]

Instead of reverting to String, maybe adding an own data type that is based on the base64 pattern could be a good solution, if validation is wanted?
(not sure what the right regex pattern would be)

[lbetz]

I thought about it, but with header and footer or without or both? Take the simple solution with strings for now.