Use spanStatus "unset" rather than "ko" when interruption cause is parallel fail fast.

Fix:

• stages with 'skipped due to earlier failure(s)' should have attribute or another status #281

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node() {
    stage('ze-parallel-stage') {
        parallel failingBranch: {
            error 'the failure'
        }, parallelBranch1: {
             sleep 5
        } ,parallelBranch2: {
             sleep 5
        } ,parallelBranch3: {
             sleep 5
        }, failFast:true
    }
}

TODO

• Unit test
• Document added attributes