Use client-go's service account token client refresh

[nrfox]

What do you want to improve?

This is a "tech debt" issue. To address: #5070, the ability to watch the service account token file and refresh Kiali's kube client/cache was added. It turns out this functionality already exists in the kubernetes go client. In order for this to work however, when you create the rest.Config you need to set the TokenFile field per: https://github.com/kubernetes/client-go/blob/84a6fe7e4032ae1b8bc03b5208e771c5f7103549/transport/round_trippers.go#L283-L302

The way that Kiali is constructing the rest.Config, Kiali does not set this.

kiali/kubernetes/client_factory.go

Lines 88 to 93 in 9a11e24

	baseConfig := rest.Config{
	Host: config.Host, // TODO: do we need this? remote cluster clients should ignore this
	TLSClientConfig: config.TLSClientConfig,
	QPS: conf.KubernetesConfig.QPS,
	Burst: conf.KubernetesConfig.Burst,
	}

Kiali should only set this for the Kiali Service Account Client but once that field is set, the client should be able to refresh itself which would simplify the client and cache.

What is the new behavior?

Token refresh works by relying on the underlying client-go to do the refresh.

[jmazzitelli]

In order for this to work however, when you create the rest.Config you need to set the TokenFile field per:

I believe you mean BearerTokenFile as declared here: https://github.com/kubernetes/client-go/blob/v12.0.0/rest/config.go#L74-L77