Gate openstack-manuals on validation script

The openstack-manuals validation script is now passing regularly,
and the doc team is ready to use it for gating.

Fixes bug 1090933

Change-Id: I2264bda33072758d43bb31fc7841bf61393d2c6f
Reviewed-on: https://review.openstack.org/27459
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins