Adding ssh_asf module, splitting ssh_asf::server_options out

This lets us have a base hash for ssh_asf::server_options while
getting the remaining field (AuthorizedKeysCommandUser or
AuthorizedKeysCommandRunAs) on a per-distro basis, so that we can
specify a different field for CentOS than everything else.
Merge pull request #4 from abayer/ssh-centos

Adding ssh_asf module, splitting ssh_asf::server_options out