Relax lxml version requirements.

This was preventing devstack from starting on Fedora 18 due to the
package python-lxml-3.2.1-1.fc18.x86_64 being too new.

This change is aligns the lxml rule with the requirements project.

Change-Id: Idd3697596dcefb830d60f8e44171b8c50e78178a
Merge "Relax lxml version requirements."