Published MasterInfo to ZK in JSON format.

As part of the support for non-libmesos linked client to
interact with the HTTP API, we also provide the Master information
stored in ZooKeeper in a way that is deserializable also for
clients that do not necessarily know how to decode a MasterInfo
protocol buffer.

This patch publishes the data in JSON format and has been tested,
summarily, with 2 masters running this patch alongside a 0.23 Master and
one each of 0.24 and 0.23 Agent nodes.

Please note this patch does not require specific unit testing, as the functionality
is already widely tested in existing `ZooKeeper*` tests, as well as `MasterDetector*` ones.

Review: https://reviews.apache.org/r/36900

Author: Marco Massenzio

src/master/contender.cpp

@@ -22,6 +22,7 @@
 
 #include <stout/check.hpp>
 #include <stout/lambda.hpp>
+#include <stout/protobuf.hpp>
 
 #include "master/constants.hpp"
 #include "master/contender.hpp"
@@ -234,13 +235,13 @@ Future<Future<Nothing> > ZooKeeperMasterContenderProcess::contend()
     delete contender;
   }
 
-  // Serialize the MasterInfo to string.
-  string data;
-  if (!masterInfo.get().SerializeToString(&data)) {
-    return Failure("Failed to serialize data to MasterInfo");
-  }
+  // Serialize the MasterInfo to JSON.
+  JSON::Object json = JSON::Protobuf(masterInfo.get());
 
-  contender = new LeaderContender(group.get(), data, master::MASTER_INFO_LABEL);
+  contender = new LeaderContender(
+      group.get(),
+      stringify(json),
+      master::MASTER_INFO_JSON_LABEL);
   candidacy = contender->contend();
   return candidacy.get();
 }