Replace exim::simple-mail-sender with a role class

Replace the tiny exim::simple-mail-sender with a new role class,
role::mail::sender. One more step towards mail.pp untanglement.

Note that this isn't exactly a role with the strict sense, but we have
no better abstraction for this and there's already a precedent within
the repository.

Change-Id: I8481f44878d7980b0f52f6cb12aeb97336fe47ba