extract_storage: dist of ANDBD must be not dev_feature* but pre-staging

extract_storage: dist of ANDBD must be not dev_feature* but pre-staging

A yet another bug on building a kernel image from a feature branch.
So far profitbricks-linux-image-storage has been built successfully.
However, its subtask building profitbricks-andbd-modules-storage
has always failed.

Its reason was apt sources list having enabled only dev-repository,
where ANDBD modules are normally not built at all. This could result
in an error case, where installation process of ANDBD module package
tries to find a bogus kernel version, such as 2.6.32.

Even though there would exist any feature branch of ANDBD that matches
exactly with the name of feature branch of linux-storage, we could not
have any way to determine at all, whether the two feature branches
should match with each other. There's no point of using $TARGET_NAME
for configuring distribution of the ANDBD module package.

Therefore the solution is to always add a pre-staging repository
to apt sources list. Then linux-storage from a feature branch can be
installed with andbd-modules from the pre-staging (develop) branch.

(Tested only under my local pbuilder environment)