Make variants.list a symlink under /etc/ganeti

This fixes #32
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 an update-helper question

Add a new snf-image/update-helper question. If the answer to this is
positive, the helper image is updated in post-install, according
to the URL specified in the snf-image/dlurl question.
Add postrm script

Add a postrm script to clean the helper files when purging.
In rules use devflow to get the version

Don't parse the version file. Use DEB_DEVFLOW_VERSION instead. If
we create a snapshot then the version file contains a wrong version