update cluster stats interface <count> attribute names to 5.x version…

[wenpos]

This PR involves the cluster stats interface:
client.ClusterStats().Do(context.Background()).Nodes.Count:
MasterOnly
DataOnly
MasterData
Client
This works fine in ES 2.x, while in 5.x the attribute names modified:
{
"count" : {
"total" : 1,
"data" : 1,
"coordinating_only" : 0,
"master" : 1,
"ingest" : 1
}
So, this PR is going to update the attribute names in branch.v5, thanks.

[olivere]

Thanks for the hint and PR. I will take a look at the whole struct in the ES Java source. I'm taking a few days off so give me a few more days before merging.

[olivere]

Confirmed. Thank you. It will roll out with 5.0.39 soon.