Add gate-openstack-manuals-validate job.

* modules/jenkins/manifests/params.pp: Define $python_lxml_package
names for both RHEL and Ubuntu.

* modules/jenkins/manifests/slave.pp: Install Python bindings for
libxml2 and libxslt, needed by the tools/validate.py script in the
openstack/openstack-manuals project.

* .../jenkins_job_builder/config/manuals.yaml
(gate-openstack-manuals-validate) New job to run tools/validate.py
for openstack/openstack-manuals project changes.

* ... /zuul/layout.yaml(jobs): Set gate-openstack-manuals-validate
as non-voting.
(openstack/openstack-manuals): Add gate-openstack-manuals-validate
to run on check and gate pipelines.

Change-Id: I0f43834dc25fa41488f795fcf1526c9b0d031a5f
Reviewed-on: https://review.openstack.org/22156
Reviewed-by: Lorin Hochstein <lorin@nimbisservices.com>
Reviewed-by: Giampaolo Lauria <lauria@us.ibm.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins
adding coverage to reddwarf's jenkins job

Change-Id: I130ce19d9745543493207984dce5b988ba4dde6a
Reviewed-on: https://review.openstack.org/22599
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins