Correct galera improvement (correct bug #12, add enh #11)

- Fix the galera root password preseed
- Fix the debian-sys-maint user creation/password change
- Add new tests for the new features (galera)
- Complete Changelog
Remove unused comment, add variables to seed template
Merge pull request #13 from sinfomicien/fix-galera-must-preseed-root-password

Correct galera improvement (correct bug #12, add enh #11)