* Removed py36 env from 'tox.ini'.
* Updated requirements.txt with relevant 'stable/rocky' branch
libraries.
* Updated test-requirements.txt with relevant 'stable/rocky' branch
libraries.
* Replaced 'mox' with 'mock' on unit tests.
* Use stestr directly instead of ostestr to run UTs, as is done in
newer upstream branches.
* Specify basepython as python2.7 for pep8, cover jobs, in case a
python3 version of tox is used.
* Added 'flake8-import-order' and kept import-order-style as 'pep8'.
* Added zuul jobs w.r.t. 'stable/rocky' release.
* Removed '-U' option from toxenv install command, for stable/rocky
& above branches compatibility.
Change-Id: I9161749fb2d8618b695815c095bdedae0251bb76
366ab2bfdb