Decouple check dependency on corev2 cipher suites

[asachs01]

On line https://github.com/sensu/http-checks/blob/master/cmd/http-get/main.go#L136, the check implements some logic that ends up referring back to corev2 cipher suites that are implemented for the backend. This really doesn't make a lot of sense to use those cipher suites when customers/users have needs to monitor sites that may be using older/weaker cipher suites. We should remove this dependency from the plugin.

[asachs01]

Solved by #24