image/image_linux: Show snap revision when fetching

[AristoChen]

Hi,

IMO, I think it would be helpful to show the revision of snaps when building Ubuntu Core image, most of the time I need to spend quite some time(depends on the download speed, it might take more than 10 mins to download kernel snap) to fetch at least 4 snaps for building image, so if somehow I made a mistake and downloading the snap with incorrect revision, I can notice that in a relative early stage and save some time since I need to build image over and over again

thanks in advance!