Instantiate new Sandbox repo for combined istio package

In support of #2273 (closed) we need a new sandbox repo instantiated.

We will be rolling the 3 following sandbox packages:

• https://repo1.dso.mil/big-bang/apps/sandbox/istio-base
• https://repo1.dso.mil/big-bang/apps/sandbox/istiod
• https://repo1.dso.mil/big-bang/apps/sandbox/istio-gateway

...into a new single istio sandbox package.

The new sandbox repo should live @ https://repo1.dso.mil/big-bang/apps/sandbox/istio

The above 3 repos will continue to live on for the time being during this transition and a separate story will be created to track the related purge task.