Add dynamic completion scaffolding to the clan `cli`.
Also add a dynamic completion mechanism for machines for commands that
have machines as their sole argument.
More intricate dynamic completions will be implemented in follow up
PR's.
Add `--regenerate` flag to `clan facts generate` which allows forcing
the generation of facts, regardless of their current existence.
Examples:
```
clan facts generate [MACHINE] --regenerate
```
or
```
clan facts generate [MACHINE] --service [SERVICE] --regenerate
```
Add `--service` flag to the `clan` cli which allows specifying a certain
service to be generated.
Example:
```
clan facts generate [MACHINE] --service [SERVICE]
```
Fixes#1395
