Use exim4-daemon-heavy on the tools mail relay

This has support for advanced lookup types like LDAP.

Class exim4 as included from role::mail::sender uses parameter
variant => light by default; this attempts to override that.

Change-Id: If5196034813c592f8b6de39690c9ca1ce18f99a6
Merge "Use exim4-daemon-heavy on the tools mail relay" into production