GUI-645: Style fixes for launch instance and launch config wizards

Fixes https://eucalyptus.atlassian.net/browse/GUI-645

@jmolo: Note that we need to keep the "Next" button for the image id search since an enter key press would submit the entire form, unexpectedly launching the instance with the entered image id.  The enter key has been disabled on that page, so we need to have a button for the image search form.