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.

(cherry picked from commit 5b1ae3eb8879f3a42989c5ee9733298870913a0e)
Merge pull request #92 from dirkmueller/release/stoney/master

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