Without this, running DevStack on an `aarch64` environment will end
up in cpu_model set to "Nehalem" and cpu_mode set to "host-passthrough"
which does not work.
This patch drops that value under aarch64 environments.
Change-Id: I30be5a388dda5ccf08718670dbb14a28a4a8a8eb
(cherry picked from commit 28bed125a2)