Pin to pip 18.1

When pip 19.0 was released, builds started failing with messages similar to Cannot uninstall 'requests'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall. When we stopped installing the python-requests-whl package, this error would move to another package, like urllib3. In the end, it seemed that pinning the pip version to 18.1 prevented these errors from happening.

deployment/packer/template.js

@@ -89,7 +89,7 @@
                 "sleep 5",
                 "sudo apt-get update -qq",
                 "sudo apt-get install python-pip python-dev libffi-dev libssl-dev -y",
-                "sudo pip install --upgrade pip",
+                "sudo pip install --upgrade pip==18.1",
                 "sudo pip install ansible==2.5.4.0",
                 "sudo /bin/sh -c 'echo {{user `version`}} > /srv/version.txt'"
             ]