Download cache is unusable if url aren't hashed

It looks even better to change the default value on Download.__init__()

And something should also be done not to cache something local.

slapos/recipe/cmmi/__init__.py

@@ -164,9 +164,9 @@ class Recipe(object):
                 return self.buildout.get(platform_part)
 
     def download_file(self, url):
-        download = Download(self.buildout['buildout'])
+        download = Download(self.buildout['buildout'], hash_name=True)
         url, _s_, md5sum = url.partition('#')
-        return download(url, md5sum=None if md5sum == '' else md5sum)
+        return download(url, md5sum=md5sum or None)
 
     def get_installed_files(self, ref_file):
         # if [buildout] has option 'prefix', then return all the files