x/build: find new cloud provider for Solaris, Illumos builders?

Now that we have SmartOS builders on Joyent in a custom image using the buildlet, let's use the Joyent API and dynamically create the containers as needed. This should be done by creating a new pool type in x/build/cmd/coordinator, similar to the GCE, reverse, and Kubernetes pool types.

This will both be cheaper (run zero when we need zero), but also let us scale from 0 to dozens as needed and let us do sharded builds and let SmartOS be a trybot. (currently we just run 2 containers all the time)

I see lots of joyent stuff at https://godoc.org/?q=joyent

/cc @davecheney @4ad