vty: Add vty_install_default() and use for the vty nodes

This adds the vty_install_default() function that is basically the
install_default() function plus the registration of the commands
'exit' and 'end'. The latter is only provided in subnodes of
ENABLED_NODE and CONFIG_NONE.

The VTY test program is extended to check these commands.

Ticket: OW#952
diff --git a/src/vty/vty.c b/src/vty/vty.c
index 696766a..8bfc35c 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -1753,7 +1753,7 @@
 	install_element(ENABLE_NODE, &terminal_monitor_cmd);
 	install_element(ENABLE_NODE, &terminal_no_monitor_cmd);
 
-	install_default(VTY_NODE);
+	vty_install_default(VTY_NODE);
 	install_element(VTY_NODE, &vty_login_cmd);
 	install_element(VTY_NODE, &no_vty_login_cmd);
 }