Create a test for each previous versions to current version (starting
from version 10, earlier versions are not really supported).

Procedure to add test for new version can be:
 * update table structure and increase version in slapos/proxy/schema.sql
 * use generate_dump.sh script to generate
   slapos/tests/test_slapproxy/database_dump_version_$NEW_VERSION.sql
   from slapos/tests/test_slapproxy/database_dump_version_$PREVIOUS_VERSION.sql
   (with computer_id "computer")
 * add TestMigrateVersion$(NEW_VERSION)ToLatest in
   slapos/tests/test_slapproxy/__init__.py
 * use generate_dump.sh script to update in-place
   slapos/tests/test_slapproxy/database_dump_version_current.sql
   (with computer_id "slaprunner")