offline upgrade test between branches

Added the test, checking the successful "major version" upgrade, i.e. switch between branches. This test ensures that Apertis is capable to upgrade the previos version to the recent. Check that update manager is able to fix all refs after successful update.

For LAVA:

• moved macroses into separate file to share them across all templates
• new macros image_url() for calculation of image URL in one place -- allows to redefine the URL with custom path via image_base_url variable
• new variables upgradable_release and upgradable_build allows to choose the deployed image version