Add --override_instance_number arg

This allows you to e.g. spin up extra compute nodes by passing
something like --override_instance_number=cp=5

It might also be useful for bootstrapping purposes. We could give
the various bootstrapping types number:0 in the env yaml, but override
it for bootstrapping.  That way, if they die, they won't get
resurrected.
Merge pull request #20 from sorenh/override_number

Add --override_instance_number arg