Fix file upload for S3 regions requiring Signature version 4

Description

Passing the `host` parameter to boto's `connect_s3` method solves the issue and should not introduce new problems. It was suggested [on StackOverflow](https://stackoverflow.com/a/29451280/41387).

Without this fix we get the following error when using an S3 bucket from `eu-central-1` region:

```
File "/edx/app/edxapp/venvs/edxapp/src/ora2/openassessment/fileupload/api.py" in remove_file
29. return backends.get_backend().remove_file(key)
File "/edx/app/edxapp/venvs/edxapp/src/ora2/openassessment/fileupload/backends/s3.py" in remove_file
49. bucket = conn.get_bucket(bucket_name)
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/boto/s3/connection.py" in get_bucket
503. return self.head_bucket(bucket_name, headers=headers)
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/boto/s3/connection.py" in head_bucket
550. response.status, response.reason, body)
Exception Value: S3ResponseError: 400 Bad Request
```

Assignee

David Ormsbee

Reporter

Open Source Pull Request Bot

Contributor Name

Silvio Tomatis

Repo

edx/edx-ora2

Customer

None

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

Educator Experiences - Content Blocks & interactives

Platform Map Area (Levels 3 & 4)

Core Course Content Blocks - Open Response Assessments

Blended Hour Utilization Percentage

None

Priority

Unset
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