Upgrade: always checkout on reclass-system branch from origin

In case of minor upgrades, when some branches already created simple
git fetch && checkout will not update them. So let's checkout
to fresh origin branch.

Change-Id: Iebb476c055d4b87cab9aab17fb72d62a410be897
Related-Prod: PROD-28587 (PROD:28587)
(cherry picked from commit b559e54d887d007ebfffef6d07baa5aa8ca2be1d)
diff --git a/upgrade-mcp-release.groovy b/upgrade-mcp-release.groovy
index de24a41..fa6e89e 100644
--- a/upgrade-mcp-release.groovy
+++ b/upgrade-mcp-release.groovy
@@ -129,6 +129,10 @@
             def updatePipelines = ''
             def updateLocalRepos = ''
             def reclassSystemBranch = ''
+            def reclassSystemBranchDefault = gitTargetMcpVersion
+            if (gitTargetMcpVersion != 'proposed') {
+                reclassSystemBranchDefault = "origin/${gitTargetMcpVersion}"
+            }
             def driteTrainParamsYaml = env.getProperty('DRIVE_TRAIN_PARAMS')
             if (driteTrainParamsYaml) {
                 def driteTrainParams = readYaml text: driteTrainParamsYaml
@@ -138,7 +142,7 @@
                 updateClusterModel = driteTrainParams.get('UPDATE_CLUSTER_MODEL', false).toBoolean()
                 updatePipelines = driteTrainParams.get('UPDATE_PIPELINES', false).toBoolean()
                 updateLocalRepos = driteTrainParams.get('UPDATE_LOCAL_REPOS', false).toBoolean()
-                reclassSystemBranch = driteTrainParams.get('RECLASS_SYSTEM_BRANCH', gitTargetMcpVersion)
+                reclassSystemBranch = driteTrainParams.get('RECLASS_SYSTEM_BRANCH', reclassSystemBranchDefault)
             } else {
                 // backward compatibility for 2018.11.0
                 saltMastURL = env.getProperty('SALT_MASTER_URL')
@@ -147,7 +151,7 @@
                 updateClusterModel = env.getProperty('UPDATE_CLUSTER_MODEL').toBoolean()
                 updatePipelines = env.getProperty('UPDATE_PIPELINES').toBoolean()
                 updateLocalRepos = env.getProperty('UPDATE_LOCAL_REPOS').toBoolean()
-                reclassSystemBranch = gitTargetMcpVersion
+                reclassSystemBranch = reclassSystemBranchDefault
             }
 
             python.setupPepperVirtualenv(venvPepper, saltMastURL, saltMastCreds)