The issue that I'm facing relates to the fact that the output has changed, meaning that if the 'Code' is different from 0 (successful) it doesn't return a dictionary with the key 'Stdout', which leads it to fail because it is looking for something that doesn't exist.

The output is the following in UOSCI:
2019-10-04 14:34:27 [INFO] ERROR: test_901_pause_resume (zaza.openstack.charm_tests.ceilometer.tests.CeilometerTest)
2019-10-04 14:34:27 [INFO] Run pause and resume tests.
2019-10-04 14:34:27 [INFO] ----------------------------------------------------------------------
2019-10-04 14:34:27 [INFO] Traceback (most recent call last):
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/openstack/charm_tests/ceilometer/tests.py", line 121, in test_901_pause_resume
2019-10-04 14:34:27 [INFO]     with self.pause_resume(self.services):
2019-10-04 14:34:27 [INFO]   File "/usr/lib/python3.5/contextlib.py", line 59, in __enter__
2019-10-04 14:34:27 [INFO]     return next(self.gen)
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/openstack/charm_tests/test_utils.py", line 283, in pause_resume
2019-10-04 14:34:27 [INFO]     pgrep_full=pgrep_full)
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/__init__.py", line 48, in _wrapper
2019-10-04 14:34:27 [INFO]     return run(_run_it())
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/__init__.py", line 36, in run
2019-10-04 14:34:27 [INFO]     return task.result()
2019-10-04 14:34:27 [INFO]   File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
2019-10-04 14:34:27 [INFO]     raise self._exception
2019-10-04 14:34:27 [INFO]   File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
2019-10-04 14:34:27 [INFO]     result = coro.send(None)
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/__init__.py", line 47, in _run_it
2019-10-04 14:34:27 [INFO]     return await f(*args, **kwargs)
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/model.py", line 988, in async_block_until_service_status
2019-10-04 14:34:27 [INFO]     await async_block_until(_check_service, timeout=timeout)
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/model.py", line 1056, in async_block_until
2019-10-04 14:34:27 [INFO]     await asyncio.wait_for(_block(), timeout, loop=loop)
2019-10-04 14:34:27 [INFO]   File "/usr/lib/python3.5/asyncio/tasks.py", line 392, in wait_for
2019-10-04 14:34:27 [INFO]     return fut.result()
2019-10-04 14:34:27 [INFO]   File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
2019-10-04 14:34:27 [INFO]     raise self._exception
2019-10-04 14:34:27 [INFO]   File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
2019-10-04 14:34:27 [INFO]     result = coro.send(None)
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/model.py", line 1050, in _block
2019-10-04 14:34:27 [INFO]     result = await c()
2019-10-04 14:34:27 [INFO]   File "/tmp/tmp.okQAPDUoyT/func-smoke/lib/python3.5/site-packages/zaza/model.py", line 981, in _check_service
2019-10-04 14:34:27 [INFO]     response_size = len(out['Stdout'].strip())
2019-10-04 14:34:27 [INFO] KeyError: 'Stdout'
2019-10-04 14:34:27 [INFO] ----------------------------------------------------------------------
2019-10-04 14:34:27 [INFO] Ran 2 tests in 186.632s
2019-10-04 14:34:27 [INFO] FAILED
2019-10-04 14:34:27 [INFO]  (errors=1)

I've also proposed the following patch to Zaza, in order to solve this problem:
https://github.com/openstack-charmers/zaza/pull/301

It is a fix but further development should be done in order to account for other possibilities that might arise from other possible values.