[r=blake-rouse][bug=1656717][author=mpontillo] Spaces API cleanup.
* Make spaces API more backward compatible with older Juju versions.
* Always return a string for the space name. The string "undefined" will be used for null spaces.
* "undefined" is now a reserved space name.
* A specifier (in practice, the name or the ID) can now be used to specify a VLAN's space.
* Remove dead code.