Set proxy on Windows VMs while provisioning

Task-number: QTQAINFRA-1669
Change-Id: Ia6b5b668051642a3729d7c7c83c69d8797045847
Reviewed-by: Heikki Halmet <heikki.halmet@qt.io>
(cherry picked from commit 54d8396d68)

coin/provisioning/common/windows/python.ps1

@@ -63,4 +63,8 @@ Add-Path "C:\Python27;C:\Python27\Scripts"
 
 C:\Python27\python.exe -m ensurepip
 # Install python virtual env
+#if ( isProxyEnabled ) {
+#    echo "Using proxy with pip"
+#    $pip_args = "--proxy=" + (getProxy)
+#}
 C:\Python27\Scripts\pip.exe install virtualenv