I believe that you are describing crown vetch, or Coronilla varia. This is a native of the Mediterranean region that is widely planted here as a ground cover and for erosion control, such as on that hillside. It can be very invasive so it needs to be planted where it can either grow freely or where it can be controlled or contained. You placed it in the right family (legume or pea/bean) when you described the leaf shape; watch for characteristic long, narrow, fingerlike seed pods to form later. It is a very attractive plant and can also be very useful in the right place. You can look at an image of crown vetch and read more about it at: Kansas Wildflowers and Grasses.
