turn on warning-is-error for sphinx build
turn on warning-is-error for sphinx build

Change-Id: I325346f580bb85ffc8834df66a1f2e4a4664dbc7
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/glance,openstack/glance,openstack/glance