keep etag of target source instead of possible_source
keep etag of target source instead of possible_source

In proxy/controllers/base.py : ResumingGetter._get_source_and_node
self.used_source_etag should be etag of target source
instead of possible_source.

Change-Id: Id6c33e79f65cb4ad9e3249957dd57dc2da8cad98
Closes-bug: #1550734

clayg/swift,tipabu/swift,tipabu/swift,clayg/swift,nadeemsyed/swift,swiftstack/swift,psachin/swift,clayg/swift,openstack/swift,prashanthpai/swift,aerwin3/swift,openstack/swift,openstack/swift,smerritt/swift,notmyname/swift,williamthegrey/swift,matthewoliver/swift,swiftstack/swift,nadeemsyed/swift,larsbutler/swift,psachin/swift,nadeemsyed/swift,matthewoliver/swift,aerwin3/swift,smerritt/swift,prashanthpai/swift,psachin/swift,hurricanerix/swift,redbo/swift,openstack/swift,clayg/swift,notmyname/swift,revoer/keystone-8.0.0,redbo/swift,matthewoliver/swift,hurricanerix/swift,tipabu/swift,psachin/swift,tipabu/swift,larsbutler/swift,nadeemsyed/swift,williamthegrey/swift,smerritt/swift,matthewoliver/swift,notmyname/swift,hurricanerix/swift,revoer/keystone-8.0.0,notmyname/swift,smerritt/swift,swiftstack/swift,hurricanerix/swift