Consolidate EKSA_VSPHERE_* Env Var Validation

[jonathanmeier5]

We currently validate EKSA_VSPHERE_* variables in two places: here and here.

We should consolidate this validation logic.

@g-gaston and I were talking recently about the fact that a lot of code pulls data from the environment, treating it as a defacto global state object. We should think about pushing that state into config objects instead. We can then run validations on the config objects as they are created.