[Config backups] Model entry is created even if upload was not successfull

[FroggyFlox]

This is another issue noticed during work on #2569, already present in our master branch.

When uploading a config backup, a related model entry is created even if the later processing steps (such as md5 checksum, for instance) fail. This leads to an incomplete and likely not functional config backup record being created.

As noted in a recent TODO...

rockstor-core/src/rockstor/storageadmin/views/config_backup.py

Line 731 in a16d948

# TODO: use ConfigBackup() instead to avoid creating the object directly and

... I think we should just "create a temporary" ConfigBackup object instead of creating that entry directly into the database from the start.