Use dev-requirements.txt in openstack/requirements

We wish to be able to use pre-release versions of some libraries during
the development cycle. The principle is that we don't want to expose
users of the stable release of those libraries to the churn that happens
during a development cycle. More background on this is described here:

  https://wiki.openstack.org/wiki/Oslo

The requirements.txt files in this repository are used for two purposes.
Firstly to describe what libraries projects are allowed to use and,
secondly, what libraries to populate our PyPI mirror with.

In the case of these development versions of libraries, they aren't
released to PyPI so we don't want them to be appear in our PyPI mirror.
So, we need to add a file which is only used for the first purpose and
ignored by the mirror-building job.

We allow dev-requirements.txt to list newer versions of libraries which
are already listed in requirements.txt so that we still get the stable
version in our mirror.

Change-Id: I176d40404adac6f7dcb2a255b9c42eb3d2c9321e
Reviewed-on: https://review.openstack.org/36128
Reviewed-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Julien Danjou <julien@danjou.info>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins