Move fas_user & keytab check into init_kerberos_ticket()
    
    Previously when the check failed, the self._kerberos_initialized
    was set to True in init_kerberos_ticket().
    
    With this commit:
    - we do the check in init_kerberos_ticket(), which looks like the place
      for checks before the actual 'kinit' is run
    - warn instead of info
    - don't set _kerberos_initialized if the check fails