Fix rspec-system.rb now that shared_contexts has been removed

Signed-off-by: Ken Barber <ken@bob.sh>
Change vagrant driver to shell out for 1.1.x compatibility

This also drops snapshot support, since Sahara doesn't work any more. Not sure
if we should bring this back in the future since the EC2 driver will be tricky
to support a 'rollback' like behaviour that is performant.

Signed-off-by: Ken Barber <ken@bob.sh>