keeping key_material in app engine_config.py

[Maximillian Dornseif]

You might be interested in a patch at https://github.com/mdornseif/approcket/tree/keepkeysecret
which moves the secret from key.py into appengine_config.py - at least
on the server site.

We generally have a policy of not checking in key material into
version control and `key.py` makes this somewhat difficult. For
`appengine_config.py` as the official supported module configuration
mechanism on AppEngine we already have policies in place so the patch
moves the secret there.

It also provides a `READONLY` option ensuring that the server side
will never accept writes to the Datastore.

The patch might be somewhat rough around the edges because it was
extracted from a much bugger changeset.

Regards

Maximillian