* Add schema_version as computed ssm document attribute
* Update the SSM document itself if the content has changed and it has a schema >= 2.0
* Add default_version as DocumentVersion in SSM doc update
* Acceptance test for updating an SSM document
* Better error handling in updating SSM documents
* Add SSM document update documentation
* Better names for SSM input params
* provider/aws: add `aws_ssm_document` resource
* provider/aws: Changes to `aws_ssm_document` post code review
The changes are things like using d.Id rather than d.Get("name").(string)
and errwrap.Wrapf rather than fmt.Errorf