Clean up env var when returning cells

ONOS_USER, ONOS_GROUP, ONOS_WEB_USER and ONOS_WEB_PASS will be reset to default value when cleaned up.
Users can override the default value by specifying desired value in local cell files.

Change-Id: I52badd0446e109fcfc7ff94779ef228e8d8276f9
diff --git a/tools/dev/bash_profile b/tools/dev/bash_profile
index 3994f3e..9e7a549 100644
--- a/tools/dev/bash_profile
+++ b/tools/dev/bash_profile
@@ -172,6 +172,7 @@
 function clearCell {
     unset ONOS_CELL ONOS_NIC ONOS_IP ONOS_APPS ONOS_BOOT_FEATURES
     unset OCI OCN OCT ONOS_INSTANCES ONOS_FEATURES
+    unset ONOS_USER ONOS_GROUP ONOS_WEB_USER ONOS_WEB_PASS
     unset $(env | sed -n 's:\(^OC[0-9]\{1,\}\)=.*:\1 :g p')
 }
 
@@ -216,10 +217,10 @@
     *)
         [ ! -f $ONOS_ROOT/tools/test/cells/$1 ] && echo "No such cell: $1" >&2 && return 1
         clearCell
-        export ONOS_USER=${ONOS_USER:-sdn}
-        export ONOS_GROUP=${ONOS_GROUP:-sdn}
-        export ONOS_WEB_USER=${ONOS_WEB_USER:-onos}
-        export ONOS_WEB_PASS=${ONOS_WEB_PASS:-rocks}
+        export ONOS_USER=sdn
+        export ONOS_GROUP=sdn
+        export ONOS_WEB_USER=onos
+        export ONOS_WEB_PASS=rocks
         export ONOS_CELL=$1
         . $ONOS_ROOT/tools/test/cells/$1
         setPrimaryInstance 1 >/dev/null