force releasenotes warnings to be treated as errors
force releasenotes warnings to be treated as errors

This patch applies the instruction in [1] to ensure that the release
notes render properly and without warnings by adding "-W" to the
sphinx-build line in tox.ini so that Sphinx can treat warnings as
errors.

  [1] [openstack-dev] [release] better linting of release notes
  http://lists.openstack.org/pipermail/openstack-dev/2015-November/080521.html

Change-Id: I64c05014e88a48b6336ce92c87da47dbe34b20d8

pshchelo/ironic,ionutbalutoiu/ironic,bacaldwell/ironic,openstack/ironic,openstack/ironic,pshchelo/ironic,SauloAislan/ironic,dims/ironic,SauloAislan/ironic,dims/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,NaohiroTamura/ironic,devananda/ironic,NaohiroTamura/ironic