Fix frontend system_proxy_delete/system_proxy_set

`OpenShift::Runtime::FrontendProxyServer#system_proxy_delete` and `OpenShift::Runtime::FrontendProxyServer#system_proxy_set`: Return values in
a consistent ordering (stdout, stderr, exit status) irrespective of whether
arguments were passed.

Previously, these methods returned the exit status as the first value instead of the last if no arguments were provided.

Fix the corresponding test cases to expect the correct ordering.

This commit fixes bug 1121238.