Update overview.md
Update overview.md

Added headnode host as a component of a HaaS installation
kylehogan/hil,meng-sun/hil,SahilTikale/switchHaaS,henn/haas,meng-sun/hil,henn/hil_sahil,henn/hil_sahil,apoorvemohan/haas,apoorvemohan/haas,kylehogan/hil,henn/hil,lokI8/haas,SahilTikale/haas,kylehogan/haas,henn/hil,CCI-MOC/haas