Assume that if VAGRANT_DEFAULT_PROVIDER is set we shouldn't install vbox tools

In Vagrant, you can set VAGRANT_DEFAULT_PROVIDER in the shell to be your default provider (i.e. "vmware_fusion"). Assume that if this is set we should not install the vbox tools.

It would be ideal to improve this logic, so that we install the tools required for the provider in question (or have multiple prepared boxes), but this works for the moment.