Fix issue with pip installing oslo.config-1.2.0

Fixes bug #1194807

Firstly, we update the oslo.config dep to 1.2.0a3 because of the issue
with namespace packages (bug #1194742).

But the main issue here is that if we ever added a dependency to our
requirements.txt that had a dependency on an older version of
oslo.config we would see the same issue thatt Nova and Quantum recently
experienced where you end up with the oslo.config 1.1.1 code installed.
This is because oslo.config>=1.1.0 gets pulled in as a transitive dep
and pip gets confused. You can reproduce with e.g.

  $> pip install \
       http://.../oslo.config-1.2.0a3.tar.gz#egg=oslo.config-1.2.0a3 \
       python-keystoneclient
  $> pip freeze | grep oslo.config
  oslo.config-1.2.0a3
  $> python -c 'from oslo.config.cfg import DeprecatedOpt'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  ImportError: cannot import name DeprecatedOpt

This is because of a bug with pip where it sees oslo.config-1.2.0a3 and
oslo.config as two unrelated things. It should strip the version part of
the egg= fragment before using it as a package name, but it doesn't.

However, we can simply use the -f/--find-links pip option in our
requirements.txt to add the tarball URL to the list of URLs considered
and also add the oslo.config>=1.2.0a3 dependency:

  $> pip install \
       -f http://.../oslo.config-1.2.0a3.tar.gz#egg=oslo.config-1.2.0a3 \
       'oslo.config>=1.2.0a3' \
       python-keystoneclient
  $> pip freeze | grep oslo.config
  oslo.config-1.2.0a3
  $> python -c 'from oslo.config.cfg import DeprecatedOpt'

This is actually exactly the semantics we want and we go to great
lengths in pbr to get these semantics while using a single tarball URL.
The only downside to this --find-links strategy is that we gain an extra
line in our requirements.txt ... but it does work around the pip bug.

Change-Id: I6f3eb5fd2c75615d9a1cae172aed859b36b27d4c
Merge "Fix issue with pip installing oslo.config-1.2.0"