Duplicate default SECRET_KEY in a variable, so that it can be overriden by localized versions

[nbachiyski]

Reproduce steps:

1. Use localized version with modified wp-config-sample.php, in which the phrase 'put your unique phrase here' is translated.
2. Check to see what is your $secret_key

Current: your secret key would be the translated version, and it will be used by many many other users

Desirable: the default value, used in wp-config-sample.php can be set also in a variable, so that localizations can override it with their default value.

[ryan]

(In [6982]) Allow overriding the default secret key. Proy. Props nbachiyski. fixes #5960