installation: allow configuring only local users (FATE#314695)

- removed LDAP/Kerberos autodetection
- 3.1.3
inst_user_first.rb rubyfication
inst_user_first.rb - code cleanup and refactoring
validation checks - pass the values as parameters

ask when username is empty (the second stage has been removed)
Merge pull request #24 from yast/only_local_users

Installation - allow only local users