Add basic error checking of vmos and vmtype command-line arguments.

Quick pair of conditionals at the start, to abort the rake task if
either the vmos or the vmtype arguments are not ones we can use.
Merge pull request #173 from fnaard/add_validation_of_vmos_and_vmtype

Add basic error checking of vmos and vmtype command-line arguments.

TODO: check other ENV vars, such as (release | latest) for PESTATUS etc.