For github repository, `git clone` on unknown repository just waits for
username/password input (which cannot happen in batch mode)

So we check by head requests, if the requests is ok, we clone, otherwise
we fail directly (raise exception and let the error callback do its bidding)

Drop the notion of repositories limit

http://docs.celeryproject.org/en/latest/whatsnew-3.1.html#preface
`You can disable this prefetching behavior by enabling the -Ofair worker option:

$ celery -A proj worker -l info -Ofair

With this option enabled the worker will only write to workers that are available for work, disabling the prefetch behavior.`

'rsync' and 'fsck' have been removed because they were redundants

Redundant with the time_rsync, time_fsck, time_cleanup keys
If they are present, the step is done...
Otherwise, not done.

We want to see if problems arise

Inside the configuration file worker-cloner.ini