As the follow up of I3aa27d4df5bb151c09003a98209c7ccf91d25828,
which enables storlet invocation over multi input objects, this
patch let docker gateway to pass metadata for extra sources to
SDaemon, so that storlet can refer user defined metadata of
the extra source objects.
Change-Id: Ic0696bcb80b581a358d4e30d044371f71acc6dc0