update pip to fix virtualenv/six issues

Change-Id: Idd6a0c26f02f32dcbd4dbab93e59af366ad42c96
diff --git a/packer/provision/basebuild.sh b/packer/provision/basebuild.sh
index 19e3876..d43525c 100644
--- a/packer/provision/basebuild.sh
+++ b/packer/provision/basebuild.sh
@@ -128,6 +128,8 @@
         # end of apt-get install list
 
     # install python modules
+    # upgrade pip or other installations may fail in unusual ways
+    pip install --upgrade pip
     pip install \
         Jinja2 \
         ansible-lint \