A prior half-torn-down topology can leave a bridge network alive under
a different name that still owns our intended subnet. Docker then
rejects our create with 'Pool overlaps with other one on this address
space', and the topology deploy fails.
Extend create_bridge_network to sweep any unused bridge whose IPAM
subnet matches the one we're about to claim (skipping networks with
running containers — those are live use).
anti
4d2e38f616