I was looking to try a bunch of mod-packs with my siblings and I'd really like to script finding and installing mod-packs onto my Ubuntu Linux server from Linode. I don't want to use the built in Linode Minecraft snapshots because I would like to be able to have a couple dozen installed and the make a simple web page with some back-end scripts to shutdown and startup different mod-packs at will but still save the worlds in separate folders.

 

Has anyone done anything similar? I know there is an API to upload and maintain mod-packs, but is there an API to get a list of available mod-packs (names, download locations, mod-pack icon/images, author, number of mods included, etc.)?

Modpacks are usually distributed by third parties. I.e. the Feed The Beast Modpacks are actually distributed by CurseForge which is a company owned by twitch, through their custom launcher. Other Modpack Distributors like Technicpack employ the same techniques. 
(Obligatory "please do not attempt to create a custom launcher")

 

None of the Distributors that I know of, have a public api to download mod-packs.  
You could of course use tools like wireshark to reverse-engineer where those launchers load their packs from, but that would be rather tedious.  

If you want to enable different modpacks on the fly, you should just write a small bash script which packs the whole server folder into a single zip file and swaps it out with another one.  
Or you can just have different folders for every mod-pack that you would like to run and only enable a single server at a time.  
You should not run different minecraft versions or even modpacks or versions of mods in the same environment, since they are usually incompatible anyways. Have them in separate folders instead.  
Zip those folders if you run into storage problems.