fixed a bug in the env name

Author: Wei Deng

Jenkinsfile

@@ -43,8 +43,8 @@ def initializeEnvironment() {
     '''
   }
 
-  if (env.CASSANDRA_VERSION.split('-')[0] == 'dse') {
-      env.DSE_FIXED_VERSION = env.CASSANDRA_VERSION.split('-')[1]
+  if (env.SERVER_VERSION.split('-')[0] == 'dse') {
+      env.DSE_FIXED_VERSION = env.SERVER_VERSION.split('-')[1]
       sh label: 'Update environment for DataStax Enterprise', script: '''#!/bin/bash -le
         cat >> ${HOME}/environment.txt << ENVIRONMENT_EOF
 CCM_PATH=${HOME}/ccm