This follows up [1] by adding an extension to allow requesting the
default subnet pool explicitly through the API thus restoring the
convenience that was lost by removing the automatic default subnetpool
fallback behavior.
ApiImpact
[1] https://review.openstack.org/#/c/279378/
Change-Id: Ifff57c0485e4727f352b2cc2bd1bdaabd0f1606b
Related-Bug: #1545199
Closes-Bug: #1547705