Install xvfb server on centos.

Previous commit ( Sudo users should be able to install/provision contrail )
caused "install -y xorg-x11-server-Xvfb" to disappear from fab task install_webui_packages
Xvfb server is required to run ui sanity.

Change-Id: Ib54d8a8e805c2b81cb0d12a36d87f179fc9c6282
Merge "Install xvfb server on centos."