Added sshd_config validation, resources for TrustedCAKeys & RevokedKeys and attribute to add iptables recipe

* Added helpers library for `sshd_config` template validation
* Added validation to `sshd_config` template before applying changes
* `sshd_config` template now refers to template resource variables instead
    of node attributes
* Added support for `TrustedCAKeys & RevokedKeys` based Auth
* Added `TrustedCAKeys` & `RevokedKeys` file resources
* Added node attribute to add recipe `openssh::iptables`