Expand 'postfix' dependency variable interface

'postfix' role gained a set of 'postfix_dependent_*' variables to
configure parts of Postfix configuration from other roles using
dependency variables. Mailman role is an example of that usage
Change init script management in 'mailman' role

Now init script is enabled using 'insserv' command instead of old
'update-rc.d' command.
Fix init.d error in 'mailman' role on Ubuntu

Ubuntu does not have 'insserv' package installed by default, so let's
use 'updat-rc.d' instead.
Merge pull request #94 from drybjed/role/postfix

Modifications in 'postfix' and 'mailman' roles