ListUsableSubnetworksRequest requests the list of usable subnetworks
available to a user for creating clusters.
.. attribute:: parent
The parent project where subnetworks are usable. Specified in
the format projects/*.
The max number of results per page that should be returned. If
the number of available results is larger than page_size,
a next_page_token is returned which can be used to get the
next page of results in subsequent requests. Acceptable values
are 0 to 500, inclusive. (Default: 500)