Here is poll3.conf

input {
  http_poller {
    urls => {
      test2 => {
        method => get
        url => "http://localhost:9200/_cluster/health"
      }
    }
    interval => 60
  }
}

filter {}

output {
  stdout { codec => rubydebug { metadata => true } }
}
==========================================================================================================
Here is poll4.conf (the only difference is the reference to the env).
input {
  http_poller {
    urls => {
      test2 => {
        method => get
        url => "http://${CONDUCTOR_IP}:9200/_cluster/health"
      }
    }
    interval => 60
  }
}

filter {}

output {
  stdout { codec => rubydebug { metadata => true } }
}


==========================================================================================================

Here is a configuration that does work with env
input {

  http_poller {
    urls => { "test2" => "http://${CONDUCTOR_IP}:8080/Fred.txt" }
    interval => 30
  }

}

filter {
}

output {
  stdout { codec => rubydebug { metadata => true } }
}

==========================================================================================================


logstash-2.3.2]$ CONDUCTOR_IP=127.0.0.1 bin/logstash --allow-env -f ~/elk/configs/poll3.conf
Settings: Default pipeline workers: 4
Pipeline main started
{
                        "cluster_name" => "elasticsearch",
                              "status" => "yellow",
                           "timed_out" => false,
                     "number_of_nodes" => 1,
    <snip>
        "response_message" => "OK",
           "times_retried" => 0
    }
}
^CSIGINT received. Shutting down the agent. {:level=>:warn}
stopping pipeline {:id=>"main"}
Pipeline main has been shutdown

==========================================================================================================


logstash-2.3.2]$ CONDUCTOR_IP=127.0.0.1 bin/logstash --allow-env -f ~/elk/configs/poll4.conf
Settings: Default pipeline workers: 4
Pipeline aborted due to error {:exception=>#<LogStash::ConfigurationError: Invalid URL http://${CONDUCTOR_IP}:9200/_cluster/health>, :backtrace=>["/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-input-http_poller-2.0.6/lib/logstash/inputs/http_poller.rb:119:in `validate_request!'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-input-http_poller-2.0.6/lib/logstash/inputs/http_poller.rb:111:in `normalize_request'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-input-http_poller-2.0.6/lib/logstash/inputs/http_poller.rb:88:in `setup_requests!'", "org/jruby/RubyHash.java:1342:in `each'", "org/jruby/RubyEnumerable.java:757:in `map'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-input-http_poller-2.0.6/lib/logstash/inputs/http_poller.rb:88:in `setup_requests!'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-input-http_poller-2.0.6/lib/logstash/inputs/http_poller.rb:79:in `register'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.2-java/lib/logstash/pipeline.rb:330:in `start_inputs'", "org/jruby/RubyArray.java:1613:in `each'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.2-java/lib/logstash/pipeline.rb:329:in `start_inputs'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.2-java/lib/logstash/pipeline.rb:180:in `start_workers'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.2-java/lib/logstash/pipeline.rb:136:in `run'", "/home/anelson/elk/logstash-2.3.2/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.2-java/lib/logstash/agent.rb:465:in `start_pipeline'"], :level=>:error}
^CSIGINT received. Shutting down the agent. {:level=>:warn}
stopping pipeline {:id=>"main"}

==========================================================================================================

logstash-2.3.2]$ CONDUCTOR_IP=127.0.0.1 bin/logstash --allow-env -f ~/elk/configs/env.conf
Settings: Default pipeline workers: 4
Pipeline main started
{
       "message" => "Fred loves Wilma, but cheats with Betty.\n",
          "tags" => [
        [0] "_jsonparsefailure"
    ],
      "@version" => "1",
    "@timestamp" => "2016-06-16T22:38:24.390Z",
     "@metadata" => {
                    "name" => "test2",
                    "host" => "localhost.localdomain",
                 "request" => {
            "method" => "get",
               "url" => "http://127.0.0.1:8080/Fred.txt"
        },
         "runtime_seconds" => 0.489,
                    "code" => 200,
        "response_headers" => {
            "content-length" => "41",
                      "date" => "Thu, 16 Jun 2016 22:38:24 GMT",
                       "age" => "0"
        },
        "response_message" => "",
           "times_retried" => 0
    }
}
^CSIGINT received. Shutting down the agent. {:level=>:warn}