Allow not setting postgres database password
Move dynamic defaults to a recipe

Currently many defaults get dynamically assigned in attributes/default.rb
depending on eg. requested postgres version and the OS.
This works fine when these are set before processing attributes, but
breaks down eg. when the postgres version is set in another recipe
(as there's no way of forcing specific load order).

Moving dynamic defaults to a recipe instead, included in postgresql::client
(which is included by the rest where relevant) makes it possible to
eg. set default postgres version in another recipe and still get
the right packages installed.