Fix LBaaS Haproxy occurs error if no member is added

If no member is added and session_persistence.type=HTTP_COOKIE,
haproxy agent would not add cookie persistence option to the backend.
Closes-Bug: #1302283

Change-Id: Ifa2564df924c2555225a749a99c705b3f1caab4a
Merge "Fix LBaaS Haproxy occurs error if no member is added"