Nova does not support a "hw:cpu_model" flavor extra
spec. This was added as part of microversion 2.47 from
WindRiver where their Titanium Cloud product does support
a hw:cpu_model flavor extra spec. We shouldn't include
this in upstream documentation lest someone be confused
and think upstream nova supports it.
Change-Id: Ie646130731fde648ab84a423024ab59f55f5daab