Drop AVAILABILITY_ZONE

By default, OpenStack will schedule you into the right availablity
zone that has capacity, hard coding this can potentially result in
not being able to provision new virtual machines.

This patch drops the hard-wired value and lets Nova on the
scheduling size pick the correct availability zone.

Signed-off-by: Mohammed Naser <mnaser@vexxhost.com>
Change-Id: Ibd3759a5f41af9ffa8f652dd47a388c5572c33d6