Optionally fetch Marathon endpoint from ZK

[drewrobb]

This gives new configuration options to allow fetching marathon endpoints as registered in zookeeper rather than hardcoded endpoints. I've used this patch for some time in our production deploy with marathon 0.11.1.

I have not tested this with https only marathon. We keep http open. I also have not tested this in conjunction with marathon user/password options.

[lclarkmichalek]

Thinking about this more, I'd really prefer to have more explicit error handling around the Endpoints method. On the http issue, if we are not going to provide an option to force HTTPS (which is a decent idea tbh), defaulting the http should be fine; if you're an attacker, you can trivially downgrade by just blocking https, if you're not, you can trivially upgrade by redirecting to https (assuming we follow redirects).

[kiambogo]

Any update on this? I'd love to see this in the latest bamboo Docker image :)

[KidkArolis]

Cleaning up old PRs, feel free to reopen if still relevant.