use Retryable for virtualbox customize + defaults

when up-ing several boxes at once, VBoxManage modifyvm gets cranky and throws random lock errors. this adds the existing retryable logic to
the customize and sane_defaults actions, both of which fire a bunch of
modifyvm commands, all of which are fine to run multiple times.
Merge pull request #2483 from andyfowler/retryable-customize

providers/virtualbox: use Retryable for Virtualbox customize & sane_defaults actions