[LIBCLOUD-544] GCE: fix adding node metadata at node creation

Passing a simple metadata dict to create_node GCE driver method doesn't work.
GCE api requires the following structure for metadata:
metadata = {"items": [{"value": "val1", "key": "key1"}, {"value": "val2", "key": "key2"}]}

This fix converts simple dict to what the API wants.