Make it possible to use local artifacts during development

[vbusch]

During todays Maas meeting, the idea of being able to use dogen to create images that included local changes (to artifacts) was desired. This would be useful during development, to make sure that the changes you are about to commit for the products, work in the images.

Can dogen have a 'local-dev' mode, where it grabs the artifacts locally? It will save people from having to drop their new artifact into the build directory, and calling 'docker build...' themselves.

[vbusch]

Added pull request #159
This covers what was discussed in #117

[goldmann]

@dbecvarik Can we review the pull request attached? Other possibility is to just use the DOGEN_SOURCES_CACHE env variable and add support to fetch from local directories, which probably is much simpler than having a plugin for it. Thoughts?

/CC @knrc

[rwngwn]

We need to decide on a way here - if upstream wants us to have overrides as a parameters then this should be done via overrides too. Also PR is outdated and needs to be updated.
Another question - do we really need this in dogen 2.0?

[goldmann]

First of all we need to have a story for overrides. Let's use #12 to talk about it, again. But this time - in a pythonic way, no -D and let's try to create requirements and design how it should work. It's still unclear for me what is the idea behind it.

I think using DOGEN_SOURCES_CACHE is not a bad thing. This can be done much faster than overrides, which could be part of some 3.X release. No, we don't need it for Dogen 2 (this particular issue was unscheduled some time ago), although I pretty much would ACK this for 3.0.

[rwngwn]

ITs against - how it will behave if you have DOGEN_SOURCE_CACHE used for like 10 artifacts and one overrided? Also how you will update cache - will you check sums for everything which is there? or just consume artifacts blindly? I think its more complex thing. We need a good stories for it first.
I also don't want to introduce some features to 2.0 which will work differently in 3.0 or will be done in a different way, because then we will be forced to have compatibility layers.