Use files get to load istio dashboard

[awhdesmond]

1. update-istio.sh will download the latest dashboards and save them locally.
2. These JSON files will be exported as a BAZEL filegroup.
3. During template rendering, we use Helm Files.Get to load the JSON files and avoid helm templating issue

bazel build //src/app_charts/base:base-cloud && bazel-bin/src/app_charts/base/app_management_test.runfiles/+non_module_deps+kubernetes_helm/helm template bazel-bin/src/app_charts/base/base-cloud-0.0.1.tgz -x templates/istio-dashboards.yaml --set-string use_istio=true | grep "name: istio-dashboard" | awk '{print $2}'


INFO: Analyzed target //src/app_charts/base:base-cloud (1205 packages loaded, 27375 targets configured).
INFO: Found 1 target...
Target //src/app_charts/base:base-cloud up-to-date:
  bazel-bin/src/app_charts/base/base-cloud-0.0.1.tgz
INFO: Elapsed time: 1.905s, Critical Path: 0.07s
INFO: 1 process: 1080 action cache hit, 1 internal.
INFO: Build completed successfully, 1 total action
istio-dashboard-istio-mesh-dashboard
istio-dashboard-istio-performance-dashboard
istio-dashboard-istio-service-dashboard
istio-dashboard-istio-workload-dashboard
istio-dashboard-pilot-dashboard

[ensonic]

Thanks a lot!