This fix addresses https://docker.atlassian.net/browse/ENGCORE-1115
Expected behaviors upon docker engine restarts:
1. IPTableEnable=true, DOCKER-USER chain present
-- no change to DOCKER-USER chain
2. IPTableEnable=true, DOCKER-USER chain not present
-- DOCKER-USER chain created and inserted top of FORWARD
chain.
3. IPTableEnable=false, DOCKER-USER chain present
-- no change to DOCKER-USER chain
the rational is that DOCKER-USER is populated
and may be used by end-user for purpose other than
filtering docker container traffic. Thus even if
IPTableEnable=false, docker engine does not touch
pre-existing DOCKER-USER chain.
4. IPTableEnable=false, DOCKER-USER chain not present
-- DOCKER-USER chain is not created.
Signed-off-by: Su Wang <su.wang@docker.com>
