Change order of common roles

With recent addition of DH params generation, 'nginx' role requires
installed openssl command. Because nginx is used as a dependency of
'apt' role (for Debian Preseed server and local APT repository) and
addition of HTTPS support in apt-get in the future, I decided to move
'pki' role before 'apt' role. It will add just a few more packages
installed directly instead via the package cache, but it shouldn't
matter in the long run - after servers are installed via PXE boot
with preseeding, they are configured to use apt cache anyway.
Added 'lsof' to list of console-related packages
Configure Mailman in Postfix conditionally

When 'mailman' capability is enabled in Postfix, but Mailman has not
been configured yet, Postfix installation cannot send mails because it
tries to lookup data from non-existent Mailman databases.

This fix will enable Mailman support in Postfix only when specific
variables are present (either 'postfix_mailman_configured' enabled using
dependency, or its counterpart in Ansible local facts), defined by
Mailman role.
Merge pull request #55 from drybjed/bugfixes

Various bugfixes