fix: renew token of vault k8s auth method

[Kryvchun]

Description
Run vault token renewer when k8s auth method is used.

References
The part of the fix to: hashicorp/envconsul#309

Relates to:

• vault: support k8s auth method #1580
• Support Vault Auth method envconsul#281
• fix: renew token of vault k8s auth method envconsul#332 (envconsul part of this fix)

Local testing

1. Configured vault in microk8s:

   vault auth enable kubernetes
   
   vault write auth/kubernetes/config \
         token_reviewer_jwt="$(cat /var/run/secrets/kubernetes.io/serviceaccount/token)" \
         kubernetes_host=https://${KUBERNETES_PORT_443_TCP_ADDR}:443 \
         kubernetes_ca_cert=@/var/run/secrets/kubernetes.io/serviceaccount/ca.crt
   
   vault write auth/kubernetes/role/vault \
        bound_service_account_names=vault \
        bound_service_account_namespaces=vault \
        ttl=10m

2. Created secret secret/passwords and configured default policy to read secret/*.
3. Port-forward vault:

   kubectl port-forward svc/vault 8200:8200 -n vault

4. Configured envconsul:

   log_level = "trace"
   
   vault {
     address = "http://localhost:8200"
     renew_token = true
   
     k8s_auth_role_name = "vault"
     k8s_service_account_token = "<JWT_FROM_SERVICE_ACCOUNT>"
     k8s_service_mount_path = "kubernetes"
   }
   
   secret {
     no_prefix = true
     path      = "secret/passwords"
   }

5. Run envconsul and checked logs:

   2023-05-18T08:25:22.357+0300 [DEBUG] envconsul: (watcher) adding vault.token
   2023-05-18T08:25:22.357+0300 [TRACE] envconsul: (watcher) vault.token starting
   2023-05-18T08:25:22.357+0300 [TRACE] envconsul: (view) vault.token starting fetch
   2023-05-18T08:25:22.358+0300 [TRACE] envconsul: vault.token: starting renewer
   2023-05-18T08:25:22.361+0300 [TRACE] envconsul: vault.token: successfully renewed
   2023-05-18T08:32:30.667+0300 [TRACE] envconsul: vault.token: successfully renewed
   2023-05-18T08:39:38.937+0300 [TRACE] envconsul: vault.token: successfully renewed
   2023-05-18T08:46:47.233+0300 [TRACE] envconsul: vault.token: successfully renewed
   2023-05-18T08:53:55.529+0300 [TRACE] envconsul: vault.token: successfully renewed
   

   Considering token TTL = 10m, everything works as expected.

[komapa]

Would love to see this merged as well!

[jnardone]

Any update on getting this in?