This changes updates the AWS Secrets manager session authentication
logic to support reading the AWS configuration file for default
credentials and region settings, if they are not provided via
environment variables.
* Modify error output a little to remove stutter "error ... : error ...`
Results before change
```
unset AWS_REGION
⇶ ~/pkg/packer build amazon-ebs_secretsmanager_shell-local.json
template: root:1:3: executing "root" at <aws_secretsmanager `packer/test/keys`
`shell`>: error calling aws_secretsmanager: Error getting secret: MissingRegion:
could not find region configuration
```
Results after change
```
unset AWS_REGION
⇶ ~/pkg/packer build amazon-ebs_secretsmanager_shell-local.json
null: output will be in this color.
==> null: Running local shell script: /tmp/packer-shell721444992
null: powershell
Build 'null' finished after 4 milliseconds 121 microseconds.
==> Wait completed after 4 milliseconds 192 microseconds
==> Builds finished. The artifacts of successful builds are:
```
* Fix typos in comments.
* Add documentation of how to use multiple keys with `aws_secretsmanager` function.
* Move `Client` struct to `secretsmanager.go` file.
Wilken Rivera
Megan Marsh
Juan Mesa