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/tests/vty/vty_test.ok b/tests/vty/vty_test.ok
index baec249..0ea2dab 100644
--- a/tests/vty/vty_test.ok
+++ b/tests/vty/vty_test.ok
@@ -1,3 +1,46 @@
 Going to test vty_cmd_string_from_valstr()
 Tested with %s-strings, resulting cmd = '[prefix%s%s%s%s%s][foo%s%s%s%s%s][sep%s%s%s%s%s][bar%s%s%s%s%s][end%s%s%s%s%s]'
+Going to test VTY node tree structure
+Going to execute 'enable'
+Returned: 0, Current node: 3 '%s# '
+Going to execute 'configure terminal'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'exit'
+Returned: 0, Current node: 3 '%s# '
+Going to execute 'configure terminal'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'end'
+Returned: 0, Current node: 3 '%s# '
+Going to execute 'configure terminal'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'log stderr'
+Returned: 0, Current node: 7 '%s(config-log)# '
+Going to execute 'exit'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'log stderr'
+Returned: 0, Current node: 7 '%s(config-log)# '
+Going to execute 'end'
+Returned: 0, Current node: 3 '%s# '
+Going to execute 'configure terminal'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'line vty'
+Returned: 0, Current node: 8 '%s(config-line)# '
+Going to execute 'exit'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'line vty'
+Returned: 0, Current node: 8 '%s(config-line)# '
+Going to execute 'end'
+Returned: 0, Current node: 3 '%s# '
+Going to execute 'configure terminal'
+Returned: 0, Current node: 4 '%s(config)# '
+Going to execute 'log stderr'
+Returned: 0, Current node: 7 '%s(config-log)# '
+Going to execute 'line vty'
+Returned: 0, Current node: 8 '%s(config-line)# '
+Going to execute 'log stderr'
+Returned: 0, Current node: 7 '%s(config-log)# '
+Going to execute 'end'
+Returned: 0, Current node: 3 '%s# '
+Going to execute 'exit'
+Returned: 0, Current node: 3 '%s# '
 All tests passed