Handle zone resource exhaustion

[rtertiaer]

In being quick, I pinned things to zones, expecting resource exhaustion to be rare. That may not be true - I cannot create a VM in my requested zone at the moment. Ensure arbitrary zones can and are used, and also handle resource exhaustion (retry with a different zone, retry with a different machine class, etc.)

[rtertiaer]

I did actually anticipate this a smidge and exposed a ZONE envvar, which makes trying again manually possible. This is still not a great user experience and could use some tidying up.