Fix tox -e releasenotes
Fix tox -e releasenotes

The releasenotes tox target fails without this patch because it
can't find sphinx-build.

This patch fixes the releasenotes tox target to use the documentation
requirements file to make sure sphinx is installed in the tox venv.

Change-Id: I55f69e1aa1b3f426cc732cc38947d70bdb0634a5

openstack/barbican,openstack/barbican