Add vmware workstation support.

Use a generic vmware model to support fusion and workstation. A generic
vmware provider will pick the appropriate provider. A warning message is
generated regarding missing plugin if vmware provider is requested and
neither fusion or workstation plugin is loaded.
If I left @provider as nil vagrant got really unhappy. Here's the with and without output comparison:

```shell
$ vagrant plugin uninstall vagrant-vmware-fusion
Uninstalling the 'vagrant-vmware-fusion' plugin...
$ vagrant status
The plugin 'vagrant-vmware-(fusion|workstation)' is not installed. Please install it first.
```

Without this line:
```shell
$ vagrant status
/Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/plugins/kernel_v2/config/vm.rb:253:in `provider': undefined method `to_sym' for nil:NilClass (NoMethodError)
	from /Users/nan/.vagrant.d/gems/gems/vagrant-config_builder-0.10.1/lib/config_builder/model/provider/vmware.rb:34:in `block in to_proc'
```

I'm wondering if this is because most people use virtualbox?