In the light of security-common-sense (see articles like http://wpresstic.com/how-to-change-wordpress-admin-username/) it is desirable to be able to set the WordPress username during the installation. Users should be able to maintain 'admin' as a username, we will not be blocking. Though, users can also just use their own username. This way, bruteforcing into our TurnkeyLinux WordPress appliances will become a tiny step harder.