feature idea: dynamic DNS setup

[thraxil]

I've been thinking about how we can avoid/fix some of the issues around hostnames, /etc/hosts, etc.

At least at Appsembler, we ought to be able to just set up .devstack.appsembler.com, delegate that to our devstack GCP project, and then use Cloud DNS to point A records at a user's instance. So, if my devstack instance is anders-at-mycomputer, I could just have anders-at-mycomputer.devstack.appsembler.com or lms.anders-at-mycomputer.devstack.appsembler.com or something automatically created and pointed at my sultan instance's IP address. Then we wouldn't have to modify the user's /etc/hosts which avoids sudo issues and cloud build weirdness. (we still need to edit ~/.ssh/config though to set ForwardAgent).

Potential issues:

• non-Appsembler sultan users would still need a way to do the equivalent setup, so that means that we either keep the /etc/hosts editing as an option, require them to set up a similar Cloud DNS setup, or make it pluggable with some other Dynamic DNS service that they want to use.
• how flexible is the devstack wrt setting a custom hostname? I've always just used whatever default (edx.devstack.lms, red.localhost, etc) was recommended and pre-configured. Will it still work if every sultan instance gets a unique hostname? Or will that conflict with how the docker compose setup works?
• DNS updates are always going to be a little slow to propagate. We could lower TTLs to 60s but that's not always respected, so users might sometimes have issues with caching.