Vault Kubernetes auth role token is not renewed

### Envconsul version
`envconsul v0.13.0 (c9c55c9)`

### Configuration
`renew_token = true` should probably be redundant like with a plaintext token, as there's no agent involved. Anyway, it makes no difference and doesn't help to resolve the issue.
```hcl
# /etc/envconsul.d/01-common.hcl
log_level = "warn"
exec {
splay = "5s"
kill_timeout = "10s"
}
max_stale = "1m"
sanitize = true
upcase = true

vault {
address = "https://vault-active.vault.svc:8200"
renew_token = true
ssl {
enabled = true
verify = true
ca_cert = "/run/secrets/kubernetes.io/serviceaccount/ca.crt"
server_name = "vault.vault.svc"
}
}

# /etc/envconsul.d/02-config.hcl
vault {
k8s_auth_role_name = "my-role"
}
secret {
path = "database/creds/my-database-role"
no_prefix = true
format = "DATABASE_{{ key }}"
}
secret {
path = "secrets/some/secret"
no_prefix = true
format = "SOME_PREFIX_{{ key }}"
}
```

### Command
```shell
envconsul -config /etc/envconsul.d -log-level=debug sleep infinity
```

### Debug output
For demo purpose I set the Vault Kubernetes role token initial TTL to 60 seconds. Max TTL is 1 hour, so it has no effect on the outcome. I also tried with different initial TTL, max TTL and period from a few minutes to a few hours, the result is always the same: once the initial TTL expires, envconsul starts to log 403s.
https://gist.github.com/dizeee/7bfa522a44ccab6852ecb8557b414eaf

### Expected behavior
Vault token is renewed before its TTL expires. It would also be nice to rotate the token when max TTL is reached.

### Actual behavior
403 errors appear in the log after the token TTL expires.

### References
- GH-274