usecase: ssl proxy server node should not require VPP

SSL Proxy user guide suggests end user uses the script to run the nginx
server. The script assumes VPP is built within the dataplane-stack repo.
However, when running the script on a non-DUT node, the user may not
have built VPP yet, as it is not needed.

So, the script breaks when the end user runs it as suggested on the
server node.

This updates the script to only require VPP to be built if the script is
running the nginx server in VPP loopback mode.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I62c7efb8cc6d30042ba62ed548f793fe52127cd8