Unable to remove/leave Agent with unreachable Node-Address

### `consul version` for both Client and Server
Client: `0.8.3`
Server: `0.8.3`

### `consul info` for both Client and Server
Client:
```
agent:
check_monitors = 0
check_ttls = 0
checks = 0
services = 0
build:
prerelease =
revision = ea2a82b
version = 0.8.3
consul:
known_servers = 3
server = false
runtime:
arch = amd64
cpu_count = 32
goroutines = 117
max_procs = 32
os = linux
version = go1.8.1
serf_lan:
encrypted = true
event_queue = 0
event_time = 219
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 223
members = 11
query_queue = 0
query_time = 18
```

Server:
```
agent:
check_monitors = 0
check_ttls = 0
checks = 1
services = 2
build:
prerelease =
revision = ea2a82b
version = 0.8.3
consul:
bootstrap = false
known_datacenters = 1
leader = true
leader_addr = [2a00:1f08:900:1::6]:8300
server = true
raft:
applied_index = 464656
commit_index = 464656
fsm_pending = 0
last_contact = 0
last_log_index = 464656
last_log_term = 4967
last_snapshot_index = 464324
last_snapshot_term = 4966
latest_configuration = [{Suffrage:Voter ID:[2a00:1f08:0900:3:0:0:0:1]:8300 Address:[2a00:1f08:0900:3:0:0:0:1]:8300} {Suffrage:Voter ID:[2a00:1f08:0900:3:0:0:0:2]:8300 Address:[2a00:1f08:0900:3:0:0:0:2]:8300} {Suffrage:Voter ID:[2a00:1f08:900:1::6]:8300 Address:[2a00:1f08:900:1::6]:8300} {Suffrage:Voter ID:[2a00:1f08:900:3::1]:8300 Address:[2a00:1f08:900:3::1]:8300} {Suffrage:Voter ID:[2a00:1f08:900:3::2]:8300 Address:[2a00:1f08:900:3::2]:8300}]
latest_configuration_index = 193962
num_peers = 4
protocol_version = 2
protocol_version_max = 3
protocol_version_min = 0
snapshot_version_max = 1
snapshot_version_min = 0
state = Leader
term = 4967
runtime:
arch = amd64
cpu_count = 8
goroutines = 275
max_procs = 8
os = linux
version = go1.8.1
serf_lan:
encrypted = true
event_queue = 0
event_time = 219
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 223
members = 11
query_queue = 0
query_time = 18
serf_wan:
encrypted = true
event_queue = 0
event_time = 1
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 46
members = 3
query_queue = 0
query_time = 1
```

### Operating system and Environment details
RedHat EL 7.3

### Description of the Issue (and unexpected/desired result)
We had to reinstall one of the servers which was running as consul client. We did not leave/force-leave the client.
During installation we did configure a wrong bind address (IPv4) for the re-installed agent.
Somehow the agent was able to connect to the IPv6-Consul-Servers anyway.
Since then all of the cluster agents show this wrong cluster member with the state alive. But is isn't alive - all agents are complaining about not beeing able to connect to the IPv4-Address.
We are not able to leave/force-leave that wrong host (bind address has been corrected in the mean-time).
When trying a force leave on some other agent B the faulty agent is stuck in state leaving on agent B (but not on Agent C for example, where it is still alive) ,until a restart of the agent B. Then the faulty agent goes back to alive on Agent B.

As a workaround we assigned a different name/id to the faulty node. It was able to join the cluster without problems.

Since the state of the orphaned agent is 'alive' i 'don't think it will be reaped automatically. Any chance to remove it manually ?

### Reproduction steps

- Uninstall Agent without leave / force leave
- Configure wrong & unreachable bind-address, but correct server-addresss to join
- start up agent
- Agent appears with wrong ip-adress in memberlist of all other nodes in DC

### Log Fragments or Link to [gist](https://gist.github.com/)
Memberlist - the first entry is wrong and definitive not alive
ceph-dc-1-01-osd-05 10.60.0.36:8301 alive client 0.8.3 2 dc-witcom-cloud
ceph-dc-1-01-osd-05-temp-02 [2aXY:1f08:900:1::ZZZ]:8301 alive client 0.8.3 2 dc-witcom-cloud

Log from same host
2017/07/11 23:04:54 [ERR] memberlist: Failed to send ping: write udp [2aXY:1f08:900:1::ZZY]:8301->10.60.0.36:8301: sendto: network is unreachable
2017/07/11 23:04:58 [ERR] memberlist: Push/Pull with ceph-dc-1-01-osd-05 failed: dial tcp 10.60.0.36:8301: getsockopt: connection refused
2017/07/11 23:05:03 [ERR] memberlist: Failed to send ping: write udp [2aXY:1f08:900:1::ZZY]:8301->10.60.0.36:8301: sendto: network is unreachable