Fix typo in Resource._pre_from_dict_check
Fix typo in Resource._pre_from_dict_check

cls.bucket.get expects key to be a string,
but instead of it may receive dict where one of the
keys will be *key*.
This problem can be found on riak backend.

Change-Id: I824889447ea229ac7005df31156afe79dc78f42b

openstack/solar,openstack/solar,openstack/solar