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
6 files changed