This basically follows what 'neutron purge' CLI is doing. And here is the
neutron client patch for that:
https://review.openstack.org/#/c/276541
Some special care has to be done to take care of auto_ptg/l2p/l3p created
under implicit workflow otherwise the CLI will report failures while deleting
those.
Also publish the purge() API thru gbpclient interface.
Change-Id: Ib1c515f0b66cf4b958472b8f56ba1a4e574e8431