portallocator: rewrite to simplify, removes race condition

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
bridge: retry any proxy set up failures until we get a successful port, defeats a race condition

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #5958 from erikh/5738-docker_port_in_use

docker port in use