This patch moves ironic-inspector to use 'stestr' test runner
via 'ostestr' wrapper.
Effective changes:
- unit tests are now being run in parallel by default
- 'py*' jobs are not running code coverage measurements,
which is left for 'cover' job only
- 'cover' job now also produces html reports consumable by upstream CI,
and fails under 90% of code coverage
- users can leverage all the ostestr/stestr features
Change-Id: I75f539457dbb1d811fdee69de52d1bcee4c6ebe6
Depends-On: Ica838db23b5714c2da1e4313e1a7ece0ca241d44
Closes-Bug: #1666232