On tripleO templates to create a Host Aggregate it's mandatory to inform availability zone

Issue

While on Horizon or on the CLI it's not mandatory to pass availability zone when creating a host aggregate:

$ nova help aggregate-create
usage: nova aggregate-create <name> [<availability-zone>]

Create a new aggregate with the specified details.

Positional arguments:
  <name>               Name of aggregate.
  <availability-zone>  The availability zone of the aggregate (optional).

While on the tripleO templates, it's mandatory:

OS::Nova::HostAggregate
[...]
Required Properties
availability_zone
Name for the availability zone.
String value expected.
Can be updated without replacement.
name
Name for the aggregate.
String value expected.
Can be updated without replacement.
[...]