Vagrant does not respect the `--provider` flag

[chrisroberts]

When the --provider flag is specified for a vagrant up command, Vagrant should use the specified provider instead of trying to get the default provider. If the specified provider is unavailable, Vagrant should return an error. For example, currently when running vagrant up --provider hyperv on a non Windows system the result is:

$ ./vagrant up one --provider hyperv

  Bringing machine 'one' up with 'virtualbox' provider...
  ...

Vagrant should not revert to bringing up the machine with virtualbox, instead it should show an error.

$ ./vagrant up one --provider hyperv   

The provider 'hyperv' that was requested to back the machine
'one' is reporting that it isn't usable on this system. The
reason is shown below:

The Hyper-V provider only works on Windows. Please try to
use another provider.