OpenStack instructions are missing

The instructions for setting up MetalLB on OpenStack (https://github.com/slateci/slateci.github.io/commit/55ba3c86cfc4f08c97fd9fa0f6d36c06250acb47) appear to have been removed (unless I am overlooking them). These are important to include, as this is a case we use fairly frequently ourselves and is often requests by sites who are interested in trying out SLATE. 