Improve the implementation of the proxy for linux

Attempt to reproduce the same behaviour found in windows, where the proxy
is defined to connect the remote_url. So, now, after finding the proxy setup
in linux, it tries to connect to the remote_url server. If it fails, it means,
that the proxy cannot be used to connect to the server. This attempt is done
just one time. And the system knows wich proxy or no-proxy to use to
connect to the script repository.

re #7571
Merge branch 'feature/7571_proxy_linux' into develop