Allow using $http_cfg_append with list of lists

Since 5a9767dcd8e0126377ca0a53178b65fac61a63d0 we are not able to use list of lists in `$http_cfg_append`.

It should be possible to include a list of lists in order to have duplicate configuration keys, like:

    nginx::http_cfg_append:
        - [include, /etc/nginx/vhosts/*.ngx]
        - [include, /etc/nginx/vhosts/*/*.ngx]

Discussion:
I reverted the validation, which is sub-optimal. However, ideally, the patch should look like:

```patch
-    validate_hash($http_cfg_append)
+    validate_hash($http_cfg_append) || validate_array($http_cfg_append)
```
but since both functions raise Puppet::Error, there is no way that I know of to make such a conditional happen.