Change snf-image/dlurl into a URL prefix

Change the debconf variable form a URL into a url prefix. The user
will be asked to specify a prefix like this one: http://cdn.synnefo.org
and the Debian configuration scripts will create the image URL out of
it by appending /snf_image_helper_image-<VERSION>.tar.xz.

The post install script has been rewritten and this commit resolves #31
debconf: Add a download-default-helper questions

Add a new snf-image/download-default-helper question. If the answer to
this is negative then ask the user if he wants to use a custom URL or
completely omit this action