Run ceilometer upgrade action in functional tests.

Switch to using proposed packages pending LP: #1813582

Closes-Bug: #1813982
Related-Bug: #1813582
Change-Id: Ie4bce16994fee7a3c4a546e0d7450dd44e92373d

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: I773bc5379de0eca8357c05084519cd1c71905d1e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

Python 3.6 is installed by default in Ubuntu 18.04 LTS.
Therefore, according to Transition Plan [1],
it'll be handy to have py36 testenv.

For more details, please check Python2 Deprecation Timeline [2]
and Python3-first Goal - Completion Criteria [3].

[1] https://wiki.ubuntu.com/Python/Python36Transition
[2] https://governance.openstack.org/tc/resolutions/20180529-python2-deprecation-timeline.html
[3] https://review.openstack.org/#/c/575933/8/goals/stein/python3-first.rst@42

Change-Id: I1706900eb43d8189f1302d73bc469269b003e329

 Change the outdated links to the latest links in README

Change-Id: I728697f9e6ef8107e3d30039d1665bc331f5c9d9