Fix lintian warning in package description.

W: globaleaks: description-synopsis-starts-with-article
Refine packer scripts.

* Do not download vagrant public key from random places on the internet, but use
  the one stored locally.
* Do not keep the output directory of the vagrant build.
* Give a proper name to the vagrant box;
* Add a not about the use of storm 0.20 in scripts.