There's no easy way to check whether the user is overwritten
latex_engine, so revert I3a9d96c4896a0d5ccc85b74562d35852a837f29f
partially and continue to always set latex_engine.
Not setting latex_engine to xelatex breaks a number of OpenStack
repositories that dependend on the default of xelatex in
openstackdocstheme.
Change-Id: I06f1d577d108fd8f16b5aad3e00725f55817ddba