Consider moving Boot source out of advanced options

From my experience, users who are not very familiar with the plugin are quite often confused by config, as they intuitively think that configure `Name` and `Label` is enough, assuming that name matches OS image name, don't check advanced options and thus don't configure boot source and then had hard time to understand where the problem is (usually thinking there's some problem with they OS image metadata or something similar).
Maybe it would be more user friendly to move `Boot source` out of Advanced options as this is typically option which has to be configured most often.
(I have no strong opinion on it, this is just my observation and suggestion to consider)