VM wizard: Add 'showStep' event

For UI plugin development, allow moving though VM wizard steps via
custom event hook, cloudStack.instanceWizard.showStep (attached to
wizard DOM object), passing the specified target step index

Example:

$wizard.trigger('cloudStack.instanceWizard.showStep', {
  index: 2 // Show step 2 of wizard
  refresh: true // (Optional) Force refresh of step if data loaded
});