Parameterizing auth-worker user and group

Using `dovecot::config::dovecotcfsingle` and selector to more easily remove these parameters if set to 'false'. Tested and this works to both enable and remove. Much cleaner than the `dovecotcfmulti`, thanks for the tip :)

> It's handy to be able to set the auth-worker user. It's even handier to be able to set the auth-worker group. If  the shadow file needs to be read by the auth-worker (for system accounts), the group can be set to 'shadow' (you could also set the user to root, but the former is preferable). This is per the [Dovecot Wiki](http://wiki2.dovecot.org/UserIds).