substitute default ns for telemetryNs in yaml for prom install

[whitneygriffith]

Please provide a description of this PR:

The telemetryNamespace works as expected except for substituting the namespace of the subjects for the ClusterRoleBinding.

subjects:
  - kind: ServiceAccount
    name: prometheus
    namespace: istio-system

Replacing all instances of namespace: istio-system in the YAML before applying the configuration solves this. I hard coded istio-system as the user can change the systemNamespace variable and istio-system is set in samples/addons/prometheus.yaml

[istio-testing]

@whitneygriffith: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
integ-helm_istio	ee3ee6f	link	false	/test integ-helm

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[keithmattix]

/retest-required