argument naming consistency

[supakeen]

This implements my ideas from #98. Each change is a separate commit so they can be dropped or modified.

[mvo5]

Thanks for the PR. This is a tricky one. I agree with the concerns. Its also a bit of a can of worms:

1. The --datadir is a bit annyoing, the only thing we currently can customize here is the repositories and its only called like this because the reporegistry.New() will always append repositories/to the dirs passed into reporegistry.New(). So we could fix reporegistry.New() to take (the direct repo config paths) and then we could rename this to --repositories. But in the future with the image defintion in yaml this will most likely point to those as well so probably worth keeping. I not super convinved by --defs though, its nice and short but lacks the hint of that its a "rootdir" for a bunch of data.
2. The --output feels a bit too generic. It could mean, file, dir or format (like we use it now). As we generally only support specifying a dir having "dir" in it seems okay. We will (hopefully) also get supprot to change the output-filenamename of the imagefile itself (instead of the current hardcoding) and then this might also become a bit confusing.
3. Using "format" instead of "output" sounds nice, I think its early enough to do it and it seems cleaner to me (and its early enough to change this still I feel)
4. blueprint vs blueprint-path is a tricky one too, the later is better in the "more-precise" sense, the former is shorter and the help explain it but maybe not enough? I slightly lean towards shorter.

Sorry, many thoughts, naming is hard.

[achilleas-k]

Not sure about --defs. We use it a lot internally, but I have no idea how (image/distro) "definitions" sounds to a user, if it sounds like what it is.