update 2.12 community build job configs for new structure

[SethTisue]

basically finishing off scala/community-build#274. but wait until scala/community-build#309 happens.

[SethTisue]

while I'm up in this, I should also add a field in Jenkins for telling dbuild just to build a single project (and whatever it depends on). and make it easy to use when running locally, too. I do this all the time locally by editing scripts/dbuild-runner.sh and inserting the project name, but I ought to be able to use an environment variable instead, and the same thing would be a time-saver on Jenkins

[SethTisue]

and I'll need to update the documentation on how to run locally. right now I'm using a command line like scala_ref="2.12.0-6df14e5-nightly" scala_build_bootstrap_opt="2.12.0-RC1" ./scripts/jobs/integrate/community-build -n

I think the scala_build_bootstrap_opt part will go away, actually, when 309 is merged

hmm. is there some way a default scala_ref can be determined?

[SethTisue]

one more thing to resolve at the same time: should there be 2.12.0 and 2.12.x community builds both? I think right now the 2.12.0 one exists in Jenkins but doesn't actually work

[SethTisue]

fixed by @adriaanm in #192. the 2.12.0 built exists now. a projects parameter exists in Jenkins.

is there some way a default scala_ref can be determined

for now I'll just document to look at a recent Jenkins run. consider later whether doing something better is worth it