[COOK-3453] Ensure entry resource is idempotent

Here's another approach to https://github.com/opscode-cookbooks/ssh_known_hosts/pull/9.

This uses ruby directly instead of using a ruby_block.  This simplifies the provider, adds whyrun support, and fixes the idempotent issue.  Also, since it is no longer using `Chef::Util::FileEdit`, it doesn't require adding a comment to the known_hosts file.

COOK-3453