[Jenkins] Get credential in pipeline

No matter what kind of program, hard-code sensitive information is not a recommend approach.

In Jenkins, credentials can store in its own credential store and fetch in pipeline.1. Getting in withCredential

withCredentials([[$class: 'UsernamePasswordMultiBinding', credentialsId:'credential_id',
                                          usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD']
                                        ]) {
echo 'userName= $USERNAME'
echo 'password= $PASSWORD'
}

Credential will store in environment variable so it can fetch in pipeline, however, value will not shown in echo.