Return the correct RRSet object in Central.create_recordset

Previously, we returned the input RecordSet, rather than the newly
created RecordSet object. This likely hasn't caused any harm, but
was inconsistent with all the other create_ methods.

Change-Id: I75342cda4e1d7a46a28d471261d51c954c333f2b
Merge "Return the correct RRSet object in Central.create_recordset"