dovecot: allow to override sieve script locations

dovecot/templates/conf.d/90-sieve.conf.j2

@@ -36,7 +36,7 @@ plugin {
   # active script symlink is located.
   # For other types: use the ';name=' parameter to specify the name of the
   # default/active script.
-  sieve = file:~/sieve;active=~/.dovecot.sieve
+  sieve = {{ dovecot_sieve }}
 
   # The default Sieve script when the user has none. This is the location of a
   # global sieve script file, which gets executed ONLY if user's personal Sieve