cert requests not handled when the original leader vault is not available

After leader unit of vault is unavailable or removed for whatever reason,
adding unit to kubernetes-master will be stuck in "Waiting for master components to start" status.

The steps to reproduce are as follows.

1. Deploy CDK with Vault HA
I used this bundle.
2. Remove or take the leader unit of vault down
$ juju run -a vault is-leader
- Stdout: |
    False
  UnitId: vault/0
- Stdout: |
    True
  UnitId: vault/1
- Stdout: |
    False
  UnitId: vault/2
$ juju remove-unit --force vault/1
3. Add unit for kubernetes-master
$ juju add-unit kubernetes-master

After a while, the added kubernetes-master unit will be stuck in "Waiting for master components to start" status.
Since the "tls_client.certs.saved" flag is not set and no certificates found under /root/cdk/,
I believe update_certs() function[0] somehow fails to retrieve the certificates from Vault when the original leader unit is not there.

$ juju run -a kubernetes-master -- "charms.reactive -p get_flags | grep tls_client.certs.saved"
- Stdout: |2
     'tls_client.certs.saved',
  UnitId: kubernetes-master/0
- ReturnCode: 1
  Stdout: ""
  UnitId: kubernetes-master/1
$ juju run -a kubernetes-master -- sudo ls -al /root/cdk
- Stdout: |
    total 52
    drwxrwx--- 4 root root 4096 Jul 12 09:22 .
    drwx------ 7 root root 4096 Jul 12 09:29 ..
    drwxr-xr-x 2 root root 4096 Jul 12 09:23 audit
    -rw-r--r-- 1 root root 61 Jul 12 09:05 basic_auth.csv
    -r--r----- 1 root root 1245 Jul 12 09:22 ca.crt
    -rw-r--r-- 1 root root 1406 Jul 12 09:22 client.crt
    -rw-r--r-- 1 root root 1678 Jul 12 09:22 client.key
    drwxr-xr-x 2 root root 4096 Jul 12 09:22 etcd
    -rw-r--r-- 1 root root 385 Jul 12 09:10 known_tokens.csv
    -rw------- 1 root root 2014 Jul 12 09:33 kubeproxyconfig
    -rw-r--r-- 1 root root 1670 Jul 12 09:22 server.crt
    -rw-r--r-- 1 root root 1674 Jul 12 09:22 server.key
    -rw------- 1 root root 1675 Jul 12 09:05 serviceaccount.key
  UnitId: kubernetes-master/0
- Stdout: |
    total 20
    drwxr-xr-x 2 root root 4096 Jul 12 09:40 .
    drwx------ 6 root root 4096 Jul 12 09:46 ..
    -rw-r--r-- 1 root root 60 Jul 12 10:11 basic_auth.csv
    -rw-r--r-- 1 root root 385 Jul 12 10:11 known_tokens.csv
    -rw-r--r-- 1 root root 1675 Jul 12 10:11 serviceaccount.key
  UnitId: kubernetes-master/1

[0] https://github.com/juju-solutions/layer-tls-client/blob/master/reactive/tls_client.py#L93