Append lock timeout to JAVA_OPTS to avoid overwrite

Change-Id: I862b20350569918f7b31b50a67938c972762e025
diff --git a/tools/package/bin/onos-service b/tools/package/bin/onos-service
index f1320a8..d3717ac 100755
--- a/tools/package/bin/onos-service
+++ b/tools/package/bin/onos-service
@@ -10,7 +10,7 @@
 # Do modify the keystore location/password and truststore location/password accordingly
 #export JAVA_OPTS="${JAVA_OPTS:--DenableNettyTLS=true -Djavax.net.ssl.keyStore=/home/ubuntu/onos.jks -Djavax.net.ssl.keyStorePassword=222222 -Djavax.net.ssl.trustStore=/home/ubuntu/onos.jks -Djavax.net.ssl.trustStorePassword=222222}"
 
-export JAVA_OPTS=-Dds.lock.timeout.milliseconds=10000
+export JAVA_OPTS=${JAVA_OPTS:--Dds.lock.timeout.milliseconds=10000}
 
 set -e  # exit on error
 set -u  # exit on undefined variable