Fix certgen script usage of ES_HOME

This commit fixes an issue with the usage of the environment variable
ES_HOME in the certgen script; the script was missing the use of $ to
obtain the value of the environment variable ES_HOME.

Relates elastic/x-pack-elasticsearch#2046

Original commit: elastic/x-pack-elasticsearch@63128db0eb

plugin/bin/x-pack/certgen

@@ -24,7 +24,7 @@ ES_HOME=`dirname "$SCRIPT"`/../..
 # make ELASTICSEARCH_HOME absolute
 ES_HOME=`cd "$ES_HOME"; pwd`
 
-source "ES_HOME/bin/elasticsearch.in.sh"
+source "$ES_HOME/bin/elasticsearch.in.sh"
 
 if [ -x "$JAVA_HOME/bin/java" ]; then
     JAVA="$JAVA_HOME/bin/java"