adding new ssl cert check for local ssl cert checking

This check takes a different approach than the current ssl_monitor plugin - this plugin requires certs to be available to the node on the node's filesystem, and for the user running scout to have read access to them. It also does not require the openssl binary to be present since it uses the openssl stdlib provided by most ruby installs. The reason for checking certs on the local filesystem is simple - not all SSL certs are available via an http/https endpoint.

It takes two arguments - `certs` and `ignore_missing`.

`certs` is a comma delimited string which contains the full path of the certificates you want to ignore

`ignore_missing` is a true/false string. "true" is the default option since we cannot have different arguments per environment in scout, so I am required to pass all of the certs I want to check in on to the plugin. The various environments will find the certs they should be looking for and ignore the ones they shouldn't care about. If you have a massive amount of certs this could get a bit hairy.

I couldn't find any documentation for the .yml file - so feel free to point me to that so I can actually know what it does.

Also, if the name of the plugin is stupid, let me know as well.