Thsi commit adds client support for managing limits in keystone.

bp unified-limits

Change-Id: I33251dbd4d3bfaf178ca86a2f5d564ac94879dd2

This change add client support for creating, reading, updating, and
deleting registered limits.

A subsequent patch will do the same for project-specific limits.

bp unified-limits
Depends-On: https://review.openstack.org/#/c/569741/
Change-Id: I6b5d106d08af53c2ad41ed3f799e9e71d370c6dd

Python3 test requirement means that the unicode type does not exist
instead we use six.unicode to get `unicode` in py27 and `str` in
python3.

Change-Id: I2cbf2e63a9d93c232b6928acf002a45b7fbcec8e

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I0637976fc3097ff5052e38cf8a9afc7a3330034f
Depends-On: I2cbf2e63a9d93c232b6928acf002a45b7fbcec8e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

According to official site,
https://packaging.python.org/guides/analyzing-pypi-package-downloads/
PyPI package download statistics is no longer maintained and thus
should be removed.

Change-Id: Ib43759f6e7edc55f13a2466a7c6098cca883df39