Templatize the authorized keys file to support 'clearing' items

This change modifies how the authorized keys file is handled as currently there is no way to 'wipe' keys from the file due to the way ssh_auth.present works. This makes the authorized keys file a managed file, and then drops the user's key in. I don't know if this is something that we do want to merge, and it should probably be reviewed and discussed, but I feel as though it does allow greater control.
Should be 600.