Add storage_cli_optional_flags to blobstore benchmarks (#605)

* Add storage_cli_optional_flags to blobstore benchmarks

* Use empty string as fallback if storage_cli_optional_flags is not set isntead of nil

Author: kathap

jobs/blobstore_benchmark/spec

@@ -51,3 +51,5 @@ properties:
   cc.stdout_logging_enabled:
     default: false
     description: "Enable logging to stdout"
+  cc.storage_cli_optional_flags:
+    description: "Storage Cli extra flags string"

jobs/blobstore_benchmark/templates/cloud_controller_ng.yml.erb

@@ -52,6 +52,10 @@
 
   cc_cfg = deep_merge_overwrite(cc_cfg, overrides)
 
+  if cc_cfg.key?("storage_cli_optional_flags")
+    cc_cfg["storage_cli_optional_flags"] = cc_cfg["storage_cli_optional_flags"].to_s
+  end
+
   %w[resource_pool buildpacks packages droplets].each do |k|
     section = cc_cfg[k]
     next unless section.is_a?(Hash)