nova: unset cpu_model on aarch64

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
1 file changed