What type of PR is this?

/kind feature

(although from some perspectives it could be considered a bug!)

What this PR does / why we need it:

Summary: Then using the load_kube_config method, the python library is (apparently?!) unable to connect securely to a cluster where the cluster API endpoints are secured with CA certificates that have been signed by an internal/external CA authority (i.e. not self-signed).

Why is this? It seems that only the cluster CA gets stored in the ~/.kube/config file, not the whole certificate chain. Therefore, the library can only resolve the chain if it is self-contained (self-signed). If it depends on additional certificates it will fail an exception something like this...

urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='k8s-api.cluster.xyz', port=6443): Max retries exceeded with url: /api/v1/namespaces/vm-nodes/serviceaccounts (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get issuer certificate (_ssl.c:1000)')))

Issue: Currently, there does not seem to be any way to control the way the REST client pool_manager is created, and it's default configuration makes it impossible (AFAICS?!) to correctly/cleanly specify the CA chain to use.

Which issue(s) this PR fixes:

Fixes #2329

Special notes for your reviewer:

N/A

Does this PR introduce a user-facing change?

Adds the ability to specify a custom CA chain for cases where the Kubernetes API is using externally-signed certificates.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

If you are using `load_kube_config` with a cluster that requires a certificate chain, you can supply a custom PoolManager that like this:

\`\`\`
    kubernetes.config.load_kube_config()
    custom_pool_manager = urllib3.PoolManager(
        cert_reqs="CERT_REQUIRED",
        ca_certs="/etc/ssl/certs/ca-certificates.crt"
    )
    api_client = kubernetes.client.ApiClient(pool_manager=custom_pool_manager)
    v1 = kubernetes.client.CoreV1Api(api_client)
\`\`\`