Add support for Python 3 requirements

This patch adds the ability to handle requirements-py3.txt and
test-requirements-py3.txt as usual.

Change-Id: Ibff0bbd182cf1027f8a3ef2278d1d226538f8222
Update python-openstacksdk from the global-requirements file

Change-Id: If05ef816e1a2b9423cfc7f0544ee922a12805a9c
pbr: update to 0.6

0.6 version adds support for multiple requirement file based on the
major Python version used. This is needed to add support for Python 3 on
some projects like oslo-incubator.

Change-Id: I868bbba20f40dbd8d57d3045921c0c566d311da6
Merge "Update python-openstacksdk from the global-requirements file"
Merge "pbr: update to 0.6"
Merge "Add support for Python 3 requirements"