Install lxml dependency

Running tempest smoke tests fail on all the XML tests due to lxml not
being installed in keystone containers.  The lxml package is no longer
a hard dependency since lxml is only required when using the (optional)
XmlBodyMiddleware middleware.

Ideally we'd want to remove XmlBodyMiddleware middleware since it's
being removed in Kilo, however I have not seen an option to disable all
the XML tests in tempest.

Closes issue #337