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.
Merge pull request #5625 from Miciah/bug-1121238-fix-frontend-system_proxy_delete-and-system_proxy_set-methods-return-value-ordering

Merged by openshift-bot