Explicit version dependency of pbr is added in requirements.txt.
Also, since pbr already landed and the old version of hacking
seems not work very well with pbr>=2, we should update it to
match global requirement. Minor flake8 issue resolved in
``proliantutils/hpssa/objects.py``.
Change-Id: I5c019ce41606b73fe12d56b871f76ef17224d4fc
Partial-Bug: #1668848
5618c0738f