pithos: rename prefix pithos_public to public

After all, the prefix is visible to users as part of the public url
they pass around, therefore 'public' is much more presentable than
pithos_public.

snf-pithos-app/pithos/api/services.py

@@ -56,7 +56,7 @@ pithos_services = {
     'pithos_public': {
         'type': 'public',
         'component': 'pithos',
-        'prefix': 'pithos_public',
+        'prefix': 'public',
         'public': False,
         'endpoints': [
             {'versionId': 'v1.0',