add libxslt1-dev to tempest required files

tempest actually needs a pretty new lxml, which means it's
going to build it from source. To do that it needs libxslt1-dev
to compile. We should be good and let devstack do this out of
the box so it works on minimal environments.

Change-Id: Ia527905c1c15fb8c6793f0ce543ad05e25a88179
(cherry picked from commit db54311552d6c1efad7d9958a539848b3aeea775)