Lock file removed even if it fails to get a lock

[Данило Шеган]

We've noticed that the lockfile gets removed in cleanup() in scripts/glance-simplestreams-sync.py even if it fails to acquire lock: since it runs atexit, cleanup is run when, on exception trying to lock, it does sys.exit().

This manifests in multiple identical images being downloaded in the initial run when there is still a per-minute cronjob: eg. I've had anything from 3 xenial and 2 trusty images to 2 xenial and 1 trusty, depending on the network speed.

What happens is this:

 1. A scripts kicks off, creates a lock file, and starts downloading images
 2. If script from 1 still hasn't finished downloading, a run after one minute finds the lockfile, aborts and removes the lockfile
 3. Two minutes in, if the script from 1 is still downloading, there is no more lockfile, so another run of the script will attempt to download the same images
 ...

The fix is simple, I'll be proposing a branch.

[Данило Шеган (danilo)]

Note, this only applies to next, the trunk already has atexit handler moved past the exception handler, which works as well.