cli: fish shell completions

cliff complete command only supports bash, but fish offers much more user friendly
completions.

cliff uses a pluggable system, but this where new shell completions can be registered,
but the plugin (CompleteFish) can not access the command manager to get more
details about options, since fish can also show help of suggestion, we want to
access the help of each option to show this during suggestions.
Also we want to make some more clever suggestions (for example slapos node start complete
with services identifiers).
For these reasons we need to register a command to replace the default complete command
and can not achieve this with simply a cliff.formatter.completion entry point.

This can be installed with:

    slapos complete > ~/.bash_completion.d/slapos
    slapos complete --shell fish > ~/.config/fish/completions/slapos.fish