Fix tempest cleanup for python3

When using python3 the file which is read and
loaded by jsonutils needs to be opened in binary
mode, otherwise, it's causing problems.
It is a known bug [1] in jsonutils from
oslo.serialization module.
The patch also adds a unit test which tests
the loading of a json file.

[1] https://bugs.launchpad.net/oslo.serialization/+bug/1515231

Related-Prod: PRODX-2516

Closes-Bug: #1806089
Change-Id: I3db71c6662d9137a4b75381de7d6dbddd3de4e09
3 files changed