Contents of stash-config.properties not restored by the backup client

More precisely Stash sends to the backup client the system.zip database backup with only a placeholder stash-config.properties in it. This is because ConfigurationBackupStep still looks for stash-config.properties in STASH_HOME not STASH_SHARED_HOME.

If a customer encounters this bug their original stash-config.properties file will still be available in the backup tar, it's just that it won't be restored (because it's the one from the system.zip file in the backup tar that is restored).