Add ignore_unstable for workflow step

Related-prod: PRODX-18117
Change-Id: I3643d62e14024f2595331f521930d1c45a230609
diff --git a/src/com/mirantis/mk/Workflow.groovy b/src/com/mirantis/mk/Workflow.groovy
index 92d086e..fc1374d 100644
--- a/src/com/mirantis/mk/Workflow.groovy
+++ b/src/com/mirantis/mk/Workflow.groovy
@@ -409,6 +409,9 @@
                     case "NOT_BUILT":
                         ignoreStepResult = step['ignore_not_built'] ?: false
                         break;
+                    case "UNSTABLE":
+                        ignoreStepResult = step['ignore_unstable'] ?: false
+                        break;
                     default:
                         ignoreStepResult = step['ignore_failed'] ?: false
                         if (ignoreStepResult && !step['skip_results'] ?: false) {