if we don't update the credentialsId field, we risk creating the credential again.

This can happen if getCredentials method is called, then the launcher is persisted later
programmatically (for example, Jenkins.getInstance() gets saved) before SSHLauncher gets
recreated via the config page.
this.username can be null, which defaults to the current OS user.

We need to pass in the guaranteed non-null version of the user name here.
avoid code duplication
upgrade process now actively sets the ID.
use the released version
[maven-release-plugin] prepare release ssh-slaves-0.24