Change VNC option to --graphics
Change VNC option to --graphics

There is no option --vnc in virt-install in RHEL8/CentOS8. As all our
hypervisors are RHEL8 bases there is no need to keep this option which
was required in RHEL6. RHEL7 had both options though --vnc was
deprecated.

From man virt-install
----%<----
Use --graphics=? to see a list of all available sub options. Complete
details at <https://libvirt.org/formatdomain.html#elementsGraphics>

This deprecates the following options: --vnc, --vncport, --vnclisten,
-k/--keymap, --sdl, --nographics
----%<----

Change-Id: Ia5fc8afe3a05b2b8ea2c52c289c413f1d35dc557

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared