Support multiple ZooKeeper servers

[androa]

In order to connect to a ZooKeeper cluster, one should/need to have several ZooKeeper hosts in the connection string. This is a comma separated host:port pairs, each corresponding to a zk server.

For example: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002".

The PR is backwards compatible with the ZOOKEEPER_HOST + ZOOKEEPER_IP settings.

[ches]

Thank you for the PR! I was thinking about this need as well, it's a welcome change.

A few requests if you wouldn't mind updating:

• Could you rebase on latest master? Apologies that I didn't review sooner, but the conflicts should be trivial to sort out.
• Please remove the commit that changes whitespace in the README, it will result in multiple single-element lists in the HTML rendered from Markdown whereas the existing format properly produces one list in GitHub's rendering. It used to look poor on the Docker Hub, it still doesn't render as one list with their parser but at least it looks legible in their new UI.
• Did you have particular reasoning for changing to relative paths in the sed command in start.sh? It seems problematic: a.) it appears incorrect, one path includes ./config and the other doesn't, and b.) if a user happens to run a shell with this container and then manually runs start.sh from there, it is going to behave badly if the user has changed directories from the default WORKDIR of /kafka and invokes start.sh with an absolute path. I'd rather avoid that possibility, and not have a brittle implicit dependency on how WORKDIR is set.

Thanks!

[androa]

I've rebased on your master and addressed your concerns, thanks for the review :)

[ches]

Looks great, thanks for the prompt revision! I squashed your last commit into your first but it's all now integrated :rocket: (af14db9952 and 38baf1f7bd)