Azure API Management update 19 April

On 19 April, we started a regular[ Azure API Management](https://aka.ms/apimrocks) service update. The update includes the following bug fixes, changes and new features: * API Management is now available in South Africa North and South Africa West regions. * API Management now refreshes certificates from Azure Key Vault every 4 hours, instead of every 24 hours. * We added a new `Authenticate-Managed-Identity` [policy](https://docs.microsoft.com/en-us/azure/api-management/api-management-authentication-policies#ManagedIdentity) that supports authenticating with an API back-end by using the managed identity of the API Management service. * We fixed an issue in Open API Specification v3 import that caused Null Reference Exceptions. We deploy updates gradually. It usually takes a week or more for every active service instance to receive the updates. * API Management * Features * Services * [ API Management](https://azure.microsoft.com/en-gb/products/api-management/)