integration/dockertestutil: wait for libnetwork settle on reconnect

DisconnectContainerFromNetwork and ReconnectContainerToNetwork
returned as soon as the docker API call completed, but libnetwork
bridge reprogramming continued for several seconds after. The HA
disconnect tests then raced and bounced between healthy and broken
bridges. Poll until the container's endpoint is gone (on
disconnect) or reconciled (on reconnect), and on the
"conflicts with existing route" surface clear the stale subnet
route from the netns and retry. Settle is now baked into the
primitive so every caller benefits.