cli: Show leader status in json output of server members

[Yethal]

Leader status is shown in regular table output but not in json output.

[hashicorp-cla]

All committers have signed the CLA.

[tgross]

Thanks @Yethal! We'll need a changelog entry for this. I would have just pushed a commit myself but you've marked the PR as unable to be edited by maintainers. If you run make cl it'll ask you for the inputs, which will result in a file with contents like this:

```release-note:improvement
cli: Add leader status to output of nomad server members -json
```

---

example of use

$ nomad server members -json
[
    {
        "Addr": "192.168.1.46",
        "DelegateCur": 4,
        "DelegateMax": 5,
        "DelegateMin": 2,
        "Name": "server0.global",
        "Port": 4648,
        "ProtocolCur": 2,
        "ProtocolMax": 5,
        "ProtocolMin": 1,
        "Status": "alive",
        "Tags": {
            "region": "global",
            "expect": "3",
            "role": "nomad",
            "dc": "dc1",
            "revision": "7fc37fa2adf1c2450b2203376f25efe66fa06655",
            "id": "e58c694c-7fff-00d9-cec0-f5da1340840e",
            "Leader": "false",
            "rpc_addr": "192.168.1.46",
            "port": "4647",
            "build": "1.5.4-dev",
            "vsn": "1",
            "raft_vsn": "3"
        }
    },
    {
        "Addr": "192.168.1.46",
        "DelegateCur": 4,
        "DelegateMax": 5,
        "DelegateMin": 2,
        "Name": "server1.global",
        "Port": 4658,
        "ProtocolCur": 2,
        "ProtocolMax": 5,
        "ProtocolMin": 1,
        "Status": "alive",
        "Tags": {
            "id": "dbd5c46f-fbc7-9c9a-33b6-2aa5fe3534da",
            "expect": "3",
            "dc": "dc1",
            "raft_vsn": "3",
            "role": "nomad",
            "build": "1.5.4-dev",
            "vsn": "1",
            "port": "4657",
            "Leader": "true",
            "rpc_addr": "192.168.1.46",
            "region": "global",
            "revision": "7fc37fa2adf1c2450b2203376f25efe66fa06655"
        }
    },
    {
        "Addr": "192.168.1.46",
        "DelegateCur": 4,
        "DelegateMax": 5,
        "DelegateMin": 2,
        "Name": "server2.global",
        "Port": 4668,
        "ProtocolCur": 2,
        "ProtocolMax": 5,
        "ProtocolMin": 1,
        "Status": "alive",
        "Tags": {
            "rpc_addr": "192.168.1.46",
            "region": "global",
            "build": "1.5.4-dev",
            "expect": "3",
            "dc": "dc1",
            "Leader": "false",
            "vsn": "1",
            "raft_vsn": "3",
            "id": "4184acc7-e1e4-504e-fa0f-c8f38510fd17",
            "role": "nomad",
            "revision": "7fc37fa2adf1c2450b2203376f25efe66fa06655",
            "port": "4667"
        }
    }
]

$ nomad server members
Name            Address       Port  Status  Leader  Raft Version  Build      Datacenter  Region
server0.global  192.168.1.46  4648  alive   false   3             1.5.4-dev  dc1         global
server1.global  192.168.1.46  4658  alive   true    3             1.5.4-dev  dc1         global
server2.global  192.168.1.46  4668  alive   false   3             1.5.4-dev  dc1         global

[tgross]

LGTM! Thanks @Yethal!

I'll get this merged once CI finishes and then it'll go out in 1.6.0