test/extended/util/url: Use curl --connect-to
test/extended/util/url: Use curl --connect-to

* test/extended/util/url/url.go (CURL): Add a new ProxyHost field.
(Through): Assign the given address to ProxyHost instead of changing the
request host or headers.
(ToShell): Use Curl's --connect-to flag to specify the proxy host.

mfojtik/origin,soltysh/origin,akram/origin,mfojtik/origin,gabemontero/origin,hroyrh/origin,jsafrane/origin,openshift/origin,hroyrh/origin,csrwng/origin,bparees/origin,jsafrane/origin,csrwng/origin,mfojtik/origin,mahak/origin,openshift/origin,sallyom/origin,hroyrh/origin,jsafrane/origin,akram/origin,bparees/origin,soltysh/origin,mahak/origin,akram/origin,deads2k/origin,csrwng/origin,openshift/origin,soltysh/origin,deads2k/origin,sallyom/origin,bparees/origin,gabemontero/origin,mahak/origin,sallyom/origin,gabemontero/origin,deads2k/origin