Fixing aws_instance schema

Some instance types have a block device by default. So when selecting
such an instance type, you will not set a config for the block device,
but the update/refresh func will notice one and update the state
nonetheless.

So in those cases the `block_device` becomes a `computed` field.
Merge pull request #714 from svanharmelen/f-fix-aws-instance-schema

provider/aws: fixing aws-instance schema