Add requests requirement capped <1.2.1.

Fixes bug 1182271.

* tools/pip-requires: The requests library released 1.2.1 today
which brings in sphinx as a new dependency. One of sphinx's
transitive dependencies is on a newer version of jinja than is
provided in the python-jinja2 package on CentOS 6, and since nova's
unit tests allow site-packages in the venv for libvirt support this
cap is needed so that gate-nova-python26 will continue to work.
Because requests is actually a dependency of python-cinderclient but
is only breaking nova's unit tests, it has to be added and come
before python-cinderclient in the pip-requires list. A separate
change has been proposed to openstack/requirements which will need
to merge first. https://review.openstack.org/29850

Change-Id: Ia198192d6231f71272d3aa4f5942828a45d86ceb