Restore use of DefaultPartSize for uploads

Restore use of DefaultPartSize for uploads where a file is provided.
Problem was introduced when the auto-calculation was done, which was
fine for upping the size if it was too small, but it had the side-effect
of decreasing it needlessly (and thus creating more transaction cost).
Use Vault.DefaultPartSize is possible

As mentioned in #1264, self.DefaultPartSize is not
considered when calculating the minimum part size.
This is an issue whenever
vault.DefaultPartSize != utils.DEFAULT_PART_SIZE.

Also added a unit test to verify that DefaultPartSize is
obeyed (provided that it can accomodate the file size).
Merge branch 'gaige-develop' into develop

Closes #1264

* gaige-develop:
  Use Vault.DefaultPartSize is possible
  Restore use of DefaultPartSize for uploads