Added host parameter to S3 connection to allow authentication in AWS Sigv4 only regions
Added host parameter to S3 connection to allow authentication in AWS Sigv4 only regions

Some AWS regions only support Signature Version 4 on their S3 endpoints.

[According to AWS documentation](https://docs.aws.amazon.com/general/latest/gr/signature-version-2.html)
these regions only support version 4 of the Signature Protocol:
  * US East (Ohio) Region
  * Canada (Central) Region
  * Asia Pacific (Mumbai) Region
  * Asia Pacific (Seoul) Region
  * EU (Frankfurt) Region
  * EU (London) Region
  * China (Beijing) Region

Without this commit, an S3 bucket from `eu-central-1` region resulted in this traceback:

```
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
```

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

edx/edx-ora2,EDUlib/edx-ora2,miptliot/edx-ora2,miptliot/edx-ora2,edx/edx-ora2,EDUlib/edx-ora2,miptliot/edx-ora2,edx/edx-ora2,EDUlib/edx-ora2,EDUlib/edx-ora2,edx/edx-ora2,miptliot/edx-ora2