Support a secondary check URL for `rolling_deploy`.

This PR supports a secondary endpoint to check during rolling deployment.  This is useful for validating that for example the load balancer sees the endpoint as up before it moves on to the next host.  Is off by default, but enabled by setting:

```ruby
set(:rolling_deploy_secondary_url, 'http://somewhere/you/care/about/')
```

Includes tests.

@spkane this may solve the problem you were experiencing.