use pip_install to install packages for tempest

Change-Id: Ia892e0bc3c8c16119686daf26595c2b82d7e27b0
diff --git a/lib/tempest b/lib/tempest
index 871e9e7..4bfdc50 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -52,7 +52,7 @@
 
     # Tempest doesn't satisfy its dependencies on its own, so
     # install them here instead.
-    sudo pip install -r $TEMPEST_DIR/tools/pip-requires
+    pip_install -r $TEMPEST_DIR/tools/pip-requires
 }