Also teach sunet-l4lb-namespace to load the nft ruleset if it exists.
While here modify the script so instead of running "once per netns
config file" we merge the interface config from each json file into the
same dict per namespace. Without this we would attempt to load the nft
ruleset twice (once per file that mentioned the namespace) or warn twice
if the file did not exist etc.
Make it able to delete addresses that are no longer in the netns config.
Also make it read one netns-base.json for hardware config which is
managed by puppet but also make it look for netns-sunet-cdn-agent.json
which is not created by puppet. This file will be generated by
sunet-cdn-agent and will include the configuration for dummy0.
