Adding vagrant dependency to intial setup instructions.
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.
Updating tar.gz download URL to include version-specific subdirectory.
Merge pull request #175 from fnaard/ticket/TRAINVM-50/add_subdirectory_to_pe_url

Updating tar.gz download URL to include version-specific subdirectory.
Adding screen and zsh to base install.
Merge pull request #176 from fnaard/ticket/TRAINVM-37/add_screen_zsh_packages_to_bootstrap

Adding screen and zsh to base install.
Merge pull request #172 from fnaard/add_vagrant_to_initial_setup

Adding vagrant dependency to intial setup instructions.
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.