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.
I don't think we should be checking installed plugins or throwing errors at this stage. Vagrant has a whole set of routines for doing this and any efforts made here will be duplicated.