res:install::fixed the username input cannot be same with usernames exsited

resolve:(temporary solution)
    In welcome.coffee,if the IsValidUsername dbus return false,
    then check FindUserByName (@input.value) .
    If return value is not "",which means the user is exsit,
    then return true.
    else then return false.

Change-Id: I6ed5af1ccbeda7bc806b5462fcf529b2cfac5f9a