Use specific purpose when using machine key to avoid derived key caching on FE
Use specific purpose when using machine key to avoid derived key caching on FE

fabiocav/azure-webjobs-sdk-script,Azure/azure-webjobs-sdk-script,Azure/azure-webjobs-sdk-script,fabiocav/azure-webjobs-sdk-script,fabiocav/azure-webjobs-sdk-script,fabiocav/azure-webjobs-sdk-script,Azure/azure-webjobs-sdk-script,Azure/azure-webjobs-sdk-script