cluster config - use secret ref for servicetoken

[dlaloue-vmware]

When configuring a cluster during install, an admin can provide a "serviceToken" to list namespaces on the target cluster. The current support is to list the token in the values.yaml file during kubeapps install/updates.

The drawback is that anytime the servicetoken must be refreshed - e.g. rotating tokens, short-life tokens (AWS has a max of 24h for tokens) - kubeapps must be updated.

The feature enhancement is to support referencing a secret that admins can more easily manage and update independently from updating kubeapps.

[stale[bot]]

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