Published MasterInfo to ZK in JSON format.
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

yuquanshan/customizedMesos,dforsyth/mesos,neilconway/mesos,chhsia0/mesos,kaysoky/mesos,gsantovena/mesos,chhsia0/mesos,craimbert/mesos,asamerh4/mesos,neilconway/mesos,abudnik/mesos,abudnik/mesos,yuquanshan/customizedMesos,reneploetz/mesos,gsantovena/mesos,jpeach/mesos,neilconway/mesos,jpeach/mesos,chhsia0/mesos,craimbert/mesos,chhsia0/mesos,verizonlabs/mesos,jpeach/mesos,zmalik/mesos,kaysoky/mesos,verizonlabs/mesos,abudnik/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,Gilbert88/mesos,asamerh4/mesos,andschwa/mesos,verizonlabs/mesos,asamerh4/mesos,andschwa/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,gsantovena/mesos,kaysoky/mesos,zmalik/mesos,andschwa/mesos,chhsia0/mesos,zmalik/mesos,zmalik/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,andschwa/mesos,kaysoky/mesos,gsantovena/mesos,Gilbert88/mesos,zmalik/mesos,yuquanshan/customizedMesos,jpeach/mesos,zmalik/mesos,verizonlabs/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,asamerh4/mesos,andschwa/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,asamerh4/mesos,neilconway/mesos,craimbert/mesos,neilconway/mesos,verizonlabs/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,yuquanshan/customizedMesos,kaysoky/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,verizonlabs/mesos,abudnik/mesos,neilconway/mesos,asamerh4/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,yuquanshan/customizedMesos,gsantovena/mesos,andschwa/mesos,craimbert/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,andschwa/mesos,abudnik/mesos,asamerh4/mesos,reneploetz/mesos,craimbert/mesos,kaysoky/mesos,zmalik/mesos