Use `tempfile` module for temporary files

[knikolla]

I would like to see all of these items being downloaded into /tmp converted to use tempfile.TemporaryFile (or tempfile.NamedTemporaryFile, if necessary) so that they are automatically cleaned up, but I think that may be for a future PR (because it will require some minor re-architecting of the code).

Originally posted by @larsks in #37 (comment)

[QuanMPhm]

@knikolla @larsks From my understanding, the script downloads the Nova DB dump from S3, optionally decompresses it, then optionally converts it to be SQLite compatible, so up to 3 DB files may be created. All 3 DB files are stored in /tmp/. Do we want to remove all 3 after the billing script finishes?

In invoicing, all the files we fetch from S3 or elsewhere remains on the system. Keeping them there was quite useful for testing local. Let me know your thoughts.

[knikolla]

@QuanMPhm yes, we want to clean up what this script downloads after it finishes.

In invoicing it makes sense to keep them there (and AFAIK they're not in /tmp/.