Catch 404 and return exists=false from GetTCPLoadBalancer

Previouly getVipByName treated 404 like any other unexpected error
return and passed it up the chain.  This caused the "if ErrNotFound then
exists=false" logic in GetTCPLoadBalancer to never fire.

This change teaches getVipByName to return ErrNotFound on a 404 server
response.

pkg/cloudprovider/openstack/openstack.go

@@ -463,6 +463,11 @@ func getVipByName(client *gophercloud.ServiceClient, name string) (*vips.Virtual
 		return true, nil
 	})
 	if err != nil {
+		if e, ok := err.(*gophercloud.UnexpectedResponseCodeError); ok {
+			if e.Actual == http.StatusNotFound {
+				return nil, ErrNotFound
+			}
+		}
 		return nil, err
 	}