Enable zlib dovecot plugin

Without this plugin we can't read existing mdbox files.

roles/mail/templates/dovecot-backend/conf.d/10-mail.conf

@@ -212,7 +212,7 @@ first_valid_uid = 100
 
 # Space separated list of plugins to load for all services. Plugins specific to
 # IMAP, LDA, etc. are added to this list in their own .conf files.
-mail_plugins = mail_crypt stats
+mail_plugins = mail_crypt stats zlib
 
 plugin {
   # Disable encryption by default, dovecot-keylookupd will then set