rename log-drain -> drain

Renamed the following commands:

* `log-drains` -> `drains`
* `log-drain-add` -> `drain-add`
* `log-drain-remove` -> `drain-remove`
* `log-drain-info` -> `drain-info`

You can compare [the old styleguide](https://hk.heroku.com/styleguide.html) to [the new styleguide](https://hkdist-staging.herokuapp.com/styleguide.html), or just look at the following relevant sections:

```bash
$ hk log-drain-add [-a <app>] <url>           # add a log drain (extra)
$ hk log-drain-info [-a <app>] <id or url>    # show info for a log drain (extra)
$ hk log-drain-remove [-a <app>] <id or url>  # remove a log drain (extra)
$ hk log-drains [-a <app>]                    # list log drains (extra)
```

```bash
$ hk drain-add [-a <app>] <url>               # add a log drain (extra)
$ hk drain-info [-a <app>] <id or url>        # show info for a log drain (extra)
$ hk drain-remove [-a <app>] <id or url>      # remove a log drain (extra)
$ hk drains [-a <app>]                        # list log drains (extra)
```

/cc @max