Introduce vagrant_halt homebin file to run on halt

If the vagrant-triggers plugin is installed, the vagrant_halt
script will fire immediately before the virual machine is halted
after issuing the `vagrant halt` command.

This script can be used to perform useful cleanup and backup tasks,
though other backup options should always be used for critical data.

Next we can add a hook to a custom script so that additional tasks
can be added without worrying about maintaining upstream compatibility