1. environs/config: add "default-space" config setting, with network.DefaultSpace as default value
2. where applicable (environs/, state/, agent/, worker/) use the "default-space" setting instead of network.DefaultSpace
3. provider/maas: add a helper to discover the default space using the primary interface of the machine
   (the one with MAC = pxc_mac of the node)
4. provider/maas: add a helper that takes the space discovered above and updates "default-space" config with it.
5. provider/maas: in Bootstrap(), wrap the returned Finalizer func with one that calls the helpers above then the orig