Improve the robustness of manuals translation scripts

Add codes to get the list of document's names in openstack manuals,
other than hard code.
Check whether the files/folders exist before taking actions,
to improve the robustness.

bug 1195180

Change-Id: I3e3d17a77920c99bed6881ac0cc460b1fce54cea
Reviewed-on: https://review.openstack.org/34700
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins