Add group:* to completions list for groups

This adds support for the asterisk in completions.

E.g.:

```
$ python supervisor/supervisorctl.py
cat:0                            RUNNING    pid 73067, uptime 4:34:42
cat:1                            RUNNING    pid 73066, uptime 4:34:42
cat:2                            RUNNING    pid 73069, uptime 4:34:42
cat:3                            RUNNING    pid 73068, uptime 4:34:42
cat:4                            RUNNING    pid 73070, uptime 4:34:42
supervisor> status ca
cat:*   cat:0   cat:1   cat:2   cat:3   cat:4
supervisor> status ca:<Tab>
cat:*   cat:0   cat:1   cat:2   cat:3   cat:4
supervisor> status cat:
supervisor> status cat:*<Tab>
supervisor> status cat:* 
```