Refactor main.cf template and attributes for greater flexibility.

**Breaking changes**:

- Attributes are namespaced as `node['postfix']`, `node['postfix']['main']`,
  and `node['postfix']['master']`.  Use corresponding setting name in main.cf
  as attribute name.
COOK-3328

* Fix case-switch typo
* Correct sasl_auth attributes, and template
* Add knob for alternate main/master templates `postfix['template_source']` - related COOK-3293 (#32)
* Update documentation
Change to template_source

* Add separate `postfix['main_template_source']` and `postfix['master_template_source']` attributes
remove irrelevant metadata entries
missing [ bracket

* missed by foodcritic *sighs*
Merge branch 'COOK-3328' of https://github.com/atomic-penguin/postfix

Conflicts:
	README.md
	attributes/default.rb