helm chart: sriov-network-metrics-exporter - allow self-signed certificates

[ianb-mp]

Using the Helm chart, with feature gate metricsExporter enabled, the metrics exporter fails to deploy unless certificates.secretName is also set. As per this comment in the upstream project, metrics exporter will generate a self-signed certificate if no TLS cert is supplied. It would be good if sriov-network-operator allowed this, rather than failing to deploy.

I split this issue from #766 as that issue covers boolean value handling.