Extend juju deploy --to format to allow specifying service name

[Brad Marshall]

A useful addition to the juju deploy --to format would be the ability to specify something like:

   $ juju deploy --to lxc:ubuntu/0

or even:

   $ juju deploy --to lxc:ubuntu

ie, specify you want to deploy a lxc to ubuntu/0, where ubuntu is a deployment of cs:ubuntu.

You can do something simliar with juju-deployer using:

   to: lxc:ubuntu=0

so it would be nice to have similar functionality in native juju.

The use case of this is when you want to deploy juju charms in a smooshed fashion, but still keep seperation. We use it extensively for deploying openstack to reduce the amount of machines required.

This is very much a wishlist bug, I have set the importance to reflect this.