buildserver: use pip instead of easy_install for caching

easy_install does not provide any download caching, while pip does. This
also moves the python module installing a shell script that takes python
packages as args.  That will allow for future uses like allowing app
metadata to include pip modules that they need.

buildserver/cookbooks/fdroidbuild-general/recipes/default.rb

@@ -1,5 +0,0 @@
-
-easy_install_package "compare-locales" do
-  options "-U"
-  action :install
-end

buildserver/provision-pip

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -e
+set -x
+
+# cache pypi downloads
+if [ -z $PIP_DOWNLOAD_CACHE ]; then
+    export PIP_DOWNLOAD_CACHE=$HOME/.pip_download_cache
+fi
+
+apt-get install --yes --no-install-recommends python-pip
+
+pip install --upgrade $@