Use a separate HAProxy map file for mapping Marathon app IDs to backends

[JayH5]

This has a few (small) advantages:

• The maps HAProxy loads are smaller
• The maps are easier to read/make sense of
• Slightly smaller/finer grained checks to see if config needs to be reloaded

[mesosphere-ci]

Can one of the admins verify this patch?

[JayH5]

I don't know if there's a tool that is used to regenerate the documentation?

[ajays20078]

It is missing a API endpoint to fetch the app maps. (Lua module to fetch app maps, similar to this) and also documentation for the API endpoint.
https://github.com/mesosphere/marathon-lb#api-endpoints

[JayH5]

I noticed that, yes. It wouldn't be too hard for me to add something like a :9090/_haproxy_getappmap endpoint, but I wanted to be sure that that was something people needed or wanted?

[brndnmtthws]

I think having the API endpoint is worthwhile. If someone plans to use this, I think there'd be an expectation that they'd be able to debug it somehow.

[brndnmtthws]

test this please

[JayH5]

Ready for review again. I've changed a few things:

• Added the /_haproxy_getappmap endpoint by rewriting getvhostmap.lua to getmaps.lua. (Sorry this is such a re-write.. took a while to wrap my head around the Lua).
• Reworked how HAProxy config is validated - before the config file was checked but with references to the old map files - now the temporary map files are written first and are set in the temp config before validation.
• Ran build-docs.sh to regenerate LongHelp.md

If there are integration tests for this stuff I'm not sure how to run them? I built & ran my changes manually and the config works as expected and the endpoints to fetch the maps work.

[brndnmtthws]

test this please