Added a note to headnode_cleanup in haas/test_common.py that documents our issues with libvirt undefine.
Added a note to headnode_cleanup in haas/test_common.py that documents our
issues with libvirt undefine.

henn/hil,henn/haas,CCI-MOC/haas,SahilTikale/switchHaaS,meng-sun/hil,kylehogan/hil,henn/hil_sahil,meng-sun/hil,henn/hil_sahil,kylehogan/hil,apoorvemohan/haas,SahilTikale/haas,kylehogan/haas,henn/hil,apoorvemohan/haas,lokI8/haas