Wip fs cmds

I realize that's your idea, but you've already run into enough "annoying" things to give up on implementing this simple idea once. ;) More than that, you're leaving off things like how we associate a given MDS instance with a filesystem (or maybe we want to let MDSes move to whatever filesystem needs them), how this is exposed to users in our reporting tools, how clients specify a filesystem to connect to (as right now they just name the monitors), how/if we segregate RADOS pools from different filesystems, etc. And while multiple maps is one way we could go forward, we might also just do some transparent mapping of "different filesystems" into folders under a root that can't see each other; that has a lot of upside as well and we haven't had any real discussion about the pros and cons of each.
There are a lot of unanswered questions with answers that might really be simple, but could also be very complicated, and exposing stuff that we need to document for users before we've answered those questions could easily involve us reproducing a bunch of documentation work or bending over backwards to try and maintain compatibility as the specification evolves.