Fix creation of auth_encryption_key attribute on migration (bnc#892512)

The template has an empty value which cannot work; we need to generate a
new value.  Since our previous migration was broken, we add a new one to
fix the setups that are already broken.
Merge pull request #91 from dirkmueller/empty-auth-encryption-key

Fix creation of auth_encryption_key attribute on migration (bnc#892512)