Add test for b2 storage

This patch adds an integration test for the b2 storage backend. This should prevent breaking the b2 storage-based backend in the future.

This is done by providing a new gitlab-CI job that makes a backup of a txt file, that contains the build number and is encrypted with the passphrase "test" to a B2 bucket. Once it's backup up, it also tries to restore the file from the bucket to a new folder. If the build ID in the file in the new folder, matches the current build ID, the backup and restore were successful.

The test should only run, when B2 credentials are set, this simplifies the forking process.

This MR also fixes #2 (closed) by reverting b2sdk to version 1.7.0