Cloning requirements repository before pip install

The commit 05aa3846a0402edc9cc49f4ba36f09592004b273 into devstack exposed a bug
where pip_install is called before the requirements repository is cloned. This
change ensures that the requirements repository exists before pip_install is
called.

Change-Id: I60b157fc98691764a69cf022852e7a95fc50cdd7
Closes-Bug: #1486304
diff --git a/stack.sh b/stack.sh
index 341df7f..639f72b 100755
--- a/stack.sh
+++ b/stack.sh
@@ -683,6 +683,11 @@
 
 # OpenStack uses a fair number of other projects.
 
+# Bring down global requirements before any use of pip_install. This is
+# necessary to ensure that the constraints file is in place before we
+# attempt to apply any constraints to pip installs.
+git_clone $REQUIREMENTS_REPO $REQUIREMENTS_DIR $REQUIREMENTS_BRANCH
+
 # Install package requirements
 # Source it so the entire environment is available
 echo_summary "Installing package prerequisites"
@@ -695,11 +700,6 @@
 
 TRACK_DEPENDS=${TRACK_DEPENDS:-False}
 
-# Bring down global requirements before any use of pip_install. This is
-# necessary to ensure that the constraints file is in place before we
-# attempt to apply any constraints to pip installs.
-git_clone $REQUIREMENTS_REPO $REQUIREMENTS_DIR $REQUIREMENTS_BRANCH
-
 # Install Python packages into a virtualenv so that we can track them
 if [[ $TRACK_DEPENDS = True ]]; then
     echo_summary "Installing Python packages into a virtualenv $DEST/.venv"