This explicitly tests connection through the load-balancer to the
gitea backend to ensure correct operation.
Additionally, it adds a check of the haproxy output to make sure the
back-ends are active (that's the srv_op_state field, c.f. [1])
[1] http://docs.haproxy.org/2.6/management.html#9.3-show%20servers%20state
Change-Id: Ia896134d6a9b6951acebfbf8b0b32a7ef8b87777
dd35b401a7