refactor authenticationmethod settings, allow user to set authenticat…

[szEvEz]

Opening again, as I failed a bit on #219 🕺

[rndmh3ro]

The tests are failing on Centos6 machines. It seems AuthenticationMethods was only introduced in OpenSSH 6.2:

• sshd(8): Added support for multiple required authentication in SSH
  protocol 2 via an AuthenticationMethods option. This option lists
  one or more comma-separated lists of authentication method names.
  Successful completion of all the methods in any list is required for
  authentication to complete. This allows, for example, requiring a
  user having to authenticate via public key or GSSAPI before they
  are offered password authentication.

See: https://www.openssh.com/releasenotes.html

Can you add a sshd_version.stdout check to the option, please?

[rndmh3ro]

Thanks John!