lava: new test for offline upgrading from other branch

Add the new template for testing the offline upgrade from previous
release. For this test the image from previous release should be used.

To deploy the proper image version new variables were added into global
list pointing to requested version of the image:
 - upgradable_url_prefix
 - upgradable_release
 - upgradable_build

The test 'aum-offline-upgrade-branch' introduce the custom path
'image_base_url' to the image for deploy, based on variables above.
This custom path is used in macros `image_url()` allowing to download
and deploy requested image version instead of the image under the test.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>