Corrected issue with keys.gnupg.net and managed mysql_version correctly

Key 1C4CBDCDCD2EFD2A couldn't be retrieved from keys.gnupg.net
(don't know why...)

$mysql_version is only accessible in init.pp, not in params.pp due to inheritance.
The logic of mysql_version has been relocated from params.pp to init.pp (not perfect but that works... :)