Improving auth method setting

Changed the way the authentication is set in this module.
*old*: more or less by hand
*new*: a new function will construct the string so Puppet can fill it into the template.

Changes:
 - added ```construct_auth_string``` function
 - added unit tests for above
 - removed ```use_kerberos_security``` and replaced it with ```use_<sec option>_auth```
 - password file will be only created if PASSWORD auth is used