Make ONOS_USER overridable

Change-Id: Ic5fef60e097da31cbffca125bc70d30ea5d5dd98
diff --git a/tools/build/envDefaults b/tools/build/envDefaults
index bfe9366..1702f08 100644
--- a/tools/build/envDefaults
+++ b/tools/build/envDefaults
@@ -31,5 +31,5 @@
 fi
 export ONOS_INSTALL_DIR="/opt/onos"     # Installation directory on remote
 export OCI="${OCI:-192.168.56.101}"     # ONOS Controller Instance
-export ONOS_USER="sdn"                  # ONOS user on remote system
+export ONOS_USER="${ONOS_USER:-sdn}"    # ONOS user on remote system
 export ONOS_PWD="rocks"                 # ONOS user password on remote system