Fix target mcp version for proposed case

Change-Id: Ic72385bbaae0e09545190ba0d5efba20217abc23
Related-prod: (PROD:26536)
(cherry picked from commit d62629c31d3b630687af13dc5916ac1d259627e2)
diff --git a/upgrade-mcp-release.groovy b/upgrade-mcp-release.groovy
index efea4a0..e67ab90 100644
--- a/upgrade-mcp-release.groovy
+++ b/upgrade-mcp-release.groovy
@@ -115,13 +115,15 @@
             }
             // end bw comp. for 2018.X => 2018.11 release
             def gitTargetMcpVersion = env.getProperty('GIT_REFSPEC')
-            if (targetMcpVersion in ['testing', 'proposed']) {
+            if (targetMcpVersion in ['nightly', 'testing']) {
                 gitTargetMcpVersion = 'master'
-                common.warningMsg("gitTargetMcpVersion has been changed to:${gitTargetMcpVersion}")
+            } else if (targetMcpVersion == 'proposed') {
+                gitTargetMcpVersion = 'proposed'
             } else if (!gitTargetMcpVersion) {
                 // backward compatibility for 2018.11.0
                 gitTargetMcpVersion = "release/${targetMcpVersion}"
             }
+            common.warningMsg("gitTargetMcpVersion has been changed to:${gitTargetMcpVersion}")
             def saltMastURL = ''
             def saltMastCreds = ''
             def upgradeSaltStack = ''