Disable vagrant-cachier caching by default, but allow to opt-in

[tknerr]

While preparing the exercises I saw quite a few vagrant-cachier related errors surfacing, especially when working with multi-vm Vagrantfiles.

Since this is highly confusing for new users, I disabled the caching that was enabled by default in ~/.vagrant.d/Vagrantfile.

You now have to explicitly opt-in to having vagrant-cachier caches enabled by default by the $GLOBAL_VAGRANT_CACHIER_ENABLED env var to any value, e.g.:

$ export GLOBAL_VAGRANT_CACHIER_ENABLED=1

To opt-out again, you have to unset the env var:

$ unset GLOBAL_VAGRANT_CACHIER_ENABLED