Construct a list of optional data volumes from the cloud profile

def _get_data_volumes(vm_):
    '''
    Construct a list of optional data volumes from the cloud profile
    '''
    ret = []
    volumes = vm_['volumes']
    for key, value in six.iteritems(volumes):
        # Verify the required 'disk_size' property is present in the cloud
        # profile config
        if 'disk_size' not in volumes[key].keys():
            raise SaltCloudConfigError(
                'The volume \'{0}\' is missing \'disk_size\''.format(key)
            )
        # Use 'HDD' if no 'disk_type' property is present in cloud profile
        if 'disk_type' not in volumes[key].keys():
            volumes[key]['disk_type'] = 'HDD'

        # Construct volume object and assign to a list.
        volume = Volume(
            name=key,
            size=volumes[key]['disk_size'],
            disk_type=volumes[key]['disk_type'],
            licence_type='OTHER'
        )

        # Set volume availability zone if defined in the cloud profile
        if 'disk_availability_zone' in volumes[key].keys():
            volume.availability_zone = volumes[key]['disk_availability_zone']

        ret.append(volume)

    return ret