Provide the ability to set s3 signature version via an option for the s3 presign subcommand

We are generating presigned s3 urls on EC2 instances using the auth details of the instance's IAM role, and providing the region as a parameter when calling `aws s3 presign ...`, so the only reason we even need aws config files for the user running the operation is to set the signature version. It would make things easier if we could simply provide an option to set the signature version rather than requiring this to be in the config file.

I realise that setting v4 as the default would be a breaking change, per https://github.com/aws/aws-cli/issues/2622#issuecomment-317130433, but hopefully providing the option wouldn't be too much of a problem (and possibly making v4 the default IF region was explicitly provided - just an idea, not a necessity)
```
$ aws --version
aws-cli/1.16.182 Python/2.7.5 Linux/3.10.0-957.21.3.el7.x86_64 botocore/1.12.172
```