mail: move backup includes in the role class

Move backup includes in the role class, instead of mail.pp. Introduce a
new empty role class for imap for this purpose.

This also fixes a bug in which /var/vmail was backed up everywhere
(although no box but sanger has it), but sanger itself wasn't actually
being backed up as it has no exim classes applied.

Change-Id: Iaee4f8939e66d991dcca58e9bd43cbbce32b4996