nova: Add attribute to configure default_schedule_zone

[vuntz]

Quoting the doc:

Availability zone to use when user doesn't specify one.

This option is used by the scheduler to determine which availability zone to place a new VM instance into if the user did not specify one at the time of VM boot request.

Possible values:

• Any string representing an availability zone name
• Default value is None. (string value)

We don't set it by default, to keep the current behavior.

Closes https://github.com/sap-oc/crowbar-openstack/issues/12

[cmurphy]

Looks fine to me too but hound is upset

[vuntz]

Changed to fix the hound comment.