Fixed ssh private key format issue

[presPetkov]

Link to issue - #299

As per the issue, the ssh private keys are in an invalid format coming from the AWS Secrets Manager. I have tried it on both Windows and Linux worker nodes and a Linux Jenkins server and the issue exists on all of them. The change suggested in this PR fixes the carriage returns so the ssh private key format is valid. I have modified the StringSupplier, as that is only used for the sshUserPrivateKey option.

Testing done

AWS Secret:

Before:

1. Jenkins can't access github using the private key:

2. When the file is taken down onto a node, it has an invalid format:

You can see the ^M in the editor

After:
1. Jenkins has access to github using the private key:

2. When the file is taken down onto a node, it has a valid format:
   

Other secrets work as expected

Username and password:

Secret string:

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[presPetkov]

This issue only seems to exist when using the AWS UI. Using the aws cli to create the secret, the issue appears to be gone. I am going to do some further testing.