Change type from String back to def so var.containsKey() method will be available

Change-Id: I8277f2b6fba4a03a8a178bf315731826a12b8817
diff --git a/vars/installVoltctl.groovy b/vars/installVoltctl.groovy
index 6737dd9..cfc525d 100644
--- a/vars/installVoltctl.groovy
+++ b/vars/installVoltctl.groovy
@@ -81,7 +81,7 @@
 
     // This logic seems odd given we branch & tag repositories
     // for release so hilight non-frozen until we know for sure.
-    String released = [
+    def released = [
         // v1.9.1 - https://github.com/opencord/voltctl/releases/tag/untagged-cd611c39178f25b95a87
         'voltha-2.12' : '1.8.45',
         'voltha-2.11' : '1.8.45',