config: postfix: make smtp-filter-timeout configurable

config: postfix: make smtp-filter-timeout configurable

accessing the value in the config-template needs to use the
item virtual method (`pmg.mail.item('filter-timeout')` instead of the
. operator, due to using kebab-case for 'filter-timeout' see [0].

the minimal value is set to 2 as we add 1 second for the rules
processing in the next patch, and postfix has 1 as minimum.

[0] http://template-toolkit.org/docs/manual/VMethods.html#section_item

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by:  Dominik Csapak <d.csapak@proxmox.com>