COOK-4086, add omnios support

* Use conf_dir attribute instead of hardcoding /etc/postfix in
  sasl_passwd recipe
* Cleanup unnecessary parts of kitchen config
* Add omnios to kitchen config
* Add 'fake' cookbook used for testing omnios
* Add omnios-specific attributes; uid attribute for postfix user is
  only used on omnios
* Use attribute for conf_dir in tests
* Add omnios specific resources to default recipe
* Append to PATH for relevant commands in sasl_passwd