Add storage_cli_optional_flags to blbostore-benchmark-schema (#4806)

Author: kathap

lib/cloud_controller/config_schemas/blobstore_benchmarks_schema.rb

@@ -49,6 +49,7 @@ class BlobstoreBenchmarksSchema < VCAP::Config
           optional(:storage_cli_config_file_buildpacks) => String,
           optional(:storage_cli_config_file_packages) => String,
           optional(:storage_cli_config_file_droplets) => String,
+          optional(:storage_cli_optional_flags) => String,
 
           db_encryption_key: enum(String, NilClass),