Detect providers

[radeksimko]

1. If there's only 1 valid provider in the Vagrantfile If there's only one way to run the VM - eg. the provided box is a VMWare box + there's related SW (eg. VMWare) installed, why couldn't Vagrant just simply choose that and not force me to define --provider when there's nothing to choose from anyway?
2. If there's more than just one option, Vagrant could either let user choose or show an error message
3. There should be a way how to define a default provider in the Vagrantfile

It seems that Vagrant is capable of some detection for a halted machine which you're not booting up for the first time (which doesn't seem to be currently too reliable), but I think Vagrant should not make users to specify obvious things.

At the end it's an automation tool.

[mitchellh]

Another issue tracking this is #3812.

Also, for #2, Vagrant chooses the "best option" or your default. The best option is defined by the provider plugins themselves, but is currently, in order: VirtualBox, Hyper-V, Docker.

We can't specify a default provider for #3 because it is system/box dependent. But we can make it like #1 that the providers specified in a Vagrantfile take higher priority.

Closing because dup, watch #3812