Support of an alternate pypi server

Currently pip will get the package from the https://pypi.python.org server.
For CI, it's a problem as Internet connection can be down,
the pypi server down, etc...

The usecase is for a company/user that maintain a local pypi mirror
and give the option to use this server instead of the official one

Change-Id: I83aac4646cb78827a92c9636d78238f8a6118642
Implements: blueprint support-local-pypi-server
diff --git a/stack.sh b/stack.sh
index bd99b5b..51854cc 100755
--- a/stack.sh
+++ b/stack.sh
@@ -652,7 +652,7 @@
 
 # Configure an appropriate python environment
 if [[ "$OFFLINE" != "True" ]]; then
-    $TOP_DIR/tools/install_pip.sh
+    PYPI_ALTERNATIVE_URL=$PYPI_ALTERNATIVE_URL $TOP_DIR/tools/install_pip.sh
 fi
 
 # Do the ugly hacks for borken packages and distros