Introduce a new minimal exim4 module

Introduce a new exim4 module that handles a very basic exim4
configuration. The module is not attempting to do anything smart with
the config files (yet) and just accepts a config argument for passing
the template.

Also move convert half of mail.pp to use the new module; most of the old
exim::* classes, including exim::roled, persist but use the exim4
hierarchy for most of their implementation.

Change-Id: I076a849434594efb20da95badc2f31d0bfcb86d4
Merge "Introduce a new minimal exim4 module" into production