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/logging_vty.c b/src/vty/logging_vty.c
index e17c7a8..64e49d7 100644
--- a/src/vty/logging_vty.c
+++ b/src/vty/logging_vty.c
@@ -677,8 +677,7 @@
 	install_element_ve(&show_alarms_cmd);
 
 	install_node(&cfg_log_node, config_write_log);
-	install_default(CFG_LOG_NODE);
-	install_element(CFG_LOG_NODE, &config_end_cmd);
+	vty_install_default(CFG_LOG_NODE);
 	install_element(CFG_LOG_NODE, &logging_fltr_all_cmd);
 	install_element(CFG_LOG_NODE, &logging_use_clr_cmd);
 	install_element(CFG_LOG_NODE, &logging_prnt_timestamp_cmd);