Available project integrations not callable from the CLI

`slug` is both required and unexpected:

```
$ python -m gitlab project-service available --project-id 1
...
__main__.py project-service available: error: the following arguments are required: --slug
```

```
$ python -m gitlab project-service available --project-id 1 --slug slack
...
TypeError: ProjectIntegrationManager.available() got an unexpected keyword argument 'slug'
```