Include role::mail::sender in role::labs::instance

Previously every labs instance had 'exim::simple-mail-sender' included
in the base node.  That class doesn't exist anymore, having been replaced
by role::mail::sender.  But, rather than replace it in the ldap
node definitions I'm just adding it here and removing exim::simple-mail-sender
from the node definitions.

Change-Id: I0877e05601c74129c8118fef63b4355e7e9cb6f3
Merge "Include role::mail::sender in role::labs::instance" into production