Log backup environment for audit purposes

Proposal

Right now it is difficult to tell why certain tasks are skipped when creating or restoring backups after the fact.

We should start logging the all relevant details at the beginning of the backup/restore log.

For example:

• Environment variables SKIP, BACKUP, STRATEGY, force, GZIP_RSYNCABLE, GITLAB_BACKUP_MAX_CONCURRENCY.
• backup_information.yml attribute skipped.