Add the 'latex_engine' option

[Chason Chan]

We need to change our typesetting engine to, e.g., "xelatex" or "lualatex"instead of plain "latex" or "pdflatex".

But the default typesetting engine of building LaTeX sources is "pdflatex". This will cause an error when generating the PDF as the following:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! Fatal fontspec error: "cannot-use-pdftex"
!
! The fontspec package requires either XeTeX or LuaTeX.
!
! You must change your typesetting engine to, e.g., "xelatex" or
! "lualatex"instead of plain "latex" or "pdflatex".
!
! See the fontspec documentation for further information.
!
! For immediate help type H <return>.
!...............................................

To fix it, configure the latex engine:

latex_engine = 'xelatex'

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-----------------------------------
Release: 1.16.2.dev17 on 2017-08-18 21:38
SHA: dddd24cc88859e90e72f16baf1e1e50184976e2d
Source: https://git.openstack.org/cgit/openstack/openstackdocstheme/tree/doc/source/index.rst
URL: https://docs.openstack.org/openstackdocstheme/latest/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/497006

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/497006
Committed: https://git.openstack.org/cgit/openstack/openstackdocstheme/commit/?id=af471ca94062a646eab56b70010c28f8d6ae9d50
Submitter: Jenkins
Branch: master

commit af471ca94062a646eab56b70010c28f8d6ae9d50
Author: chenxing <[e-mail]>
Date: Thu Aug 24 12:21:57 2017 +0800

    Update the configuration of generating a PDF document

    Change-Id: Ib02619173fb60ae834562ef54df6118f8390fc4a
    Closes-Bug: #1712726

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/openstackdocstheme 1.17.0 release.