TestEgressGatewayTls various issues

[howardjohn]

https://prow.istio.io/view/gs/istio-prow/logs/integ-security-multicluster_istio_postsubmit/1536855749987143680 example failure

Secret is not supplied by SDS because the file is not found. This is not found because the volume isn't mounted, despite being in the config.

Also our dumping logic seems suspect. I think somewhere along they way we started duplicate dumping: https://gcsweb.istio.io/gcs/istio-prow/logs/integ-security-multicluster_istio_postsubmit/1536855749987143680/artifacts/security-filebased-tls-origination-/TestEgressGatewayTls/Mutual_TLS_origination_from_egress_gateway_to_https_endpoint/_test_context/

cc @aryan16 for the SDS part, @nmittler for the dump part (just wondering if it i due to the recent namespace changes)

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