Fix AWS region vs zone

We were specifying a region, but naming it as a zone in util.sh

The zone matters just as much as the region, e.g. for EBS volumes.

We also change the config to require a Zone, not a Region.
But we fallback to get the information from the metadata service.

cluster/aws/templates/salt-master.sh

@@ -28,7 +28,7 @@ EOF
 
 cat <<EOF > /etc/aws.conf
 [Global]
-Region = ${AWS_ZONE}
+Zone = ${AWS_ZONE}
 EOF
 
 # Auto accept all keys from minions that try to join