Cleanup nova cell configuration before config it

Some old configuration(such as, LOG_COLOR config) will remain
if we don't cleanup.

So, we should cleanup the configuration before we config it.

Change-Id: I7aff609dadf3acba13a36894614b35005f51280d
diff --git a/lib/nova b/lib/nova
index ea0d2f7..50deeeb 100644
--- a/lib/nova
+++ b/lib/nova
@@ -589,6 +589,8 @@
             local vhost
             conf=$(conductor_conf $i)
             vhost="nova_cell${i}"
+            # clean old conductor conf
+            rm -f $conf
             iniset $conf database connection `database_connection_url nova_cell${i}`
             iniset $conf conductor workers "$API_WORKERS"
             iniset $conf DEFAULT debug "$ENABLE_DEBUG_LOG_LEVEL"