Integrated databags into PGD Cookbook

A new file ./databags/pgd/pgd_secrets.json has been added which is an
encrypted databag full of throw-away data (info which was already in
./attributes/default.rb) has been added. The cookbook has been updated
to read data out of the encrypted databag.

Since the 'secret' data is mostly to do with the database information the
only files which have been updated drastically are the default and database
recipes as well as the settings.py template.

The throwaway encrypted_data_bag_secret and encrypted databag are included
in ths commit.

refs #11
why set both `server_user` and `db_user` ?