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.
Updating vmos and vmtype validation to ignore when they're null.
Merge pull request #177 from fnaard/do_not_validate_null_vmos_or_vmtype

Updating vmos and vmtype validation to ignore when they're null.
Fixing typo in %post to use /bin/true.
Merge pull request #178 from fnaard/ticket/TRAINVM-52/fix_kickstart_path_for_bin_true

Fixing typo in %post to use /bin/true.