EuiForm should render to `form`

Currently the `EuiForm` component renders a `div` as container element. This is slightly confusing, I would have expected it to render an html `<form>` element, so `onSubmit` on `EuiForm` works:

```tsx
<EuiForm onSubmit={sendToServer}>
   ...
   <EuiButton fill type="submit">Submit form</EuiButton>
</EuiForm>
```