Don't install pyvirtualdisplay/selenium if ui flag isn't set

pyvirtualdisplay selenium pkgs were getting appended to pkgs string twice for centos when ui flag was set causing ui sanity to fail for centos.

Change-Id: I99c784daa08180bde9cb11c0e68a9abe34285e9a
Merge "Don't install pyvirtualdisplay/selenium if ui flag isn't set"