add_group module: strip trailing/leading whitespace from comma-separated group names

Something like this:

```
add_host: hostname=localhost groupname="mygroup, secondgroup"
```

...will result in a group being created with a leading space: 

```
ok: [localhost] => {"new_groups": ["mygroup", " secondgroup"], "new_host": "localhost"}
```

This patch strips trailing and leading whitespace from group names specified, which is probably almost always what the user means.