Fix custom postfix attributes, and improve specs to cover them

The custom postfix TLS attributes - in particular the CA file path
- were not being applied because of the changes in postfix that
moved calculation of dynamic auth/tls attributes to recipes. This
is because postfix redefines those attributes as default, which
overrides our default attributes.

For now, our attributes have to be normal. It may however be
possible to remove some and refactor along the lines of postfix
cookbook.
Remove temporary fix for opscode-cookbooks/postfix#69

This is now fixed in the upstream cookbook.
Refactor postfix attribute config to take advantage postfix::_attributes

Postfix now compiles the various dependent configs - eg for sasl_auth
and TLS - in the attributes recipe rather than in its own attrribute
definitions. This allows us to simplify our own attributes as postfix
will build the relevant dynamic config for us.

Left the specs in to avoid future unpleasantness if the postfix
defaults change, so this won't affect default behaviour but makes
it simpler to override.