Rename Installation to Instance

Follows the portal: the noun is "instance" everywhere the app says it —
UI strings, CLI output, error details, docs and comments. The wire keys
(`instance_id`, `instance_token`) and the hub route this calls move with it.

An existing cloud.json is adopted rather than refused: without the key
alias the dataclass fails to parse, which the caller swallows and reads as
"never enrolled" instead of "reconnect".

`instance_key` on a node type becomes `target_key`. It means the outside
thing a node points at, which is a different sense of the word, and keeping
both would put two meanings of "instance" in one codebase.

README.md

@@ -29,7 +29,7 @@ pip install fluksio
 fluksio serve
 ```
 
-Then, head over to [fluksio.com](https://fluksio.com), sign up and add a new installation.
+Then, head over to [fluksio.com](https://fluksio.com), sign up and add a new instance.
 Using the code provided, run
 
 ```sh