Fixed tiny bug with _meta handling in external inventory scripts

This is a trivial fix for the bug I raised earlier today https://github.com/ansible/ansible/issues/3884 - ansible_ssh_host was not effective when returned in _meta from an external inventory script.  In fact I'm not sure _meta was working at all before this change!  First pull request (ever - let alone ansible) so please educate me if something's incorrect.  Thanks!