AWS_S3_HOST environment variable no longer available
In the [What's New - Other enhancements](https://pandas.pydata.org/pandas-docs/version/0.21/whatsnew.html#whatsnew-0180-enhancements-other) section from the 18.0 release it says that you can define the environment variable `AWS_S3_HOST`. 

Therefore, when setting this environment variable, together with `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` I expected the following code to work.

```python
import pandas as pd
df = pd.DataFrame(dict(a=[1, 2, 3], b=[4, 5, 6]))
df.to_csv(fs.open('s3://bucket/key', mode='w'))
```

The script crashes with:

```
Exception ignored in: <bound method S3File.__del__ of <S3File bucket/key>>
Traceback (most recent call last):
  File "/home/xxx/.local/share/virtualenvs/xxx/lib/python3.6/site-packages/s3fs/core.py", line 1518, in __del__
    self.close()
  File "/home/xxx/.local/share/virtualenvs/xxx/lib/python3.6/site-packages/s3fs/core.py", line 1496, in close
    raise_from(IOError('Write failed: %s' % self.path), e)
  File "<string>", line 3, in raise_from
OSError: Write failed: bucket/key
```
This is the exact same error as when I don't specify the variable `AWS_S3_HOST` at all.

I managed to get a working solution by directly interacting directly with `s3fs`:
```python
import os
import pandas as pd
import s3fs

fs = s3fs.S3FileSystem(client_kwargs={'endpoint_url': os.environ["AWS_S3_HOST"]})
df = pd.DataFrame(dict(a=[1, 2, 3], b=[4, 5, 6]))
df.to_csv(fs.open('s3://bucket/key', mode='w'))
```

Looks like this feature got (accidentally?) removed in `https://github.com/pandas-dev/pandas/commit/dc4b0708f36b971f71890bfdf830d9a5dc019c7b#diff-a37b395bed03f0404dec864a4529c97dL94`, when they switched from `boto` to `s3fs`.
Is it planned to support this environment variable again?
I'm sure that many companies, e.g. like the one I work for, hosts their own s3 server, e.g. with Minio, and don't use Amazon.