Ubuntu 18.04 dovecot namespace inbox fix

Fix based on https://github.com/serghey-rodin/vesta/pull/1669/files

install/ubuntu/18.04/dovecot/dovecot.conf

@@ -3,7 +3,7 @@ listen = *, ::
 base_dir = /var/run/dovecot/
 !include conf.d/*.conf
 
-namespace {
+namespace inbox {
     type = private
     separator = /
     prefix =