juju add-model does not support specifying constraints

[Ian Booth]

As per bug 1961396, juju add-model does not support specifying model constraints.

I'm not sure about the "default to host arch" bit of the comment though.

https://bugs.launchpad.net/juju/+bug/1961396/comments/3

"I think the fix here, is to allow adding constraints when adding a model. If the constraints for arch is empty, then use the os.HostArch to set it as default. We won't have broken anything for homogeneous deployments and we'll have a more sensible architecture. Heterogeneous deployments are still in the same position they're always have been.

If the user wants to change it they can either specify it with add-model command or if nothing is deployed, you can always use juju set-model-constraints."