New site variable giving the control net gateway router's MAC address.

How we get this value to the nodes in general is still a question,
but we use this currently to pass to per-experiment firewalls so that
they can be use it in rules.

sql/sitevars-create.sql

@@ -66,3 +66,4 @@ INSERT INTO sitevariables VALUES ('general/open_showexplist',NULL,'','Allow memb
 INSERT INTO sitevariables VALUES ('swap/swapout_command',NULL,'','Command to run in admin MFS on each node of an experiment at swapout time. Runs as swapout user.');
 INSERT INTO sitevariables VALUES ('swap/swapout_command_failaction',NULL,'warn','What to do if swapout command fails (warn == continue, fail == fail swapout).');
 INSERT INTO sitevariables VALUES ('swap/swapout_command_timeout',NULL,'120','Time (in seconds) to allow for command completion');
+INSERT INTO sitevariables VALUES ('node/gw_mac','00:b0:8e:84:69:34','','MAC address of the control net router');