Resolve LBCORE-1820 "Cache slot config in main"

With this change, the main job is going to record the slot configuration files directly to the CouchDB matching documents, and checkout and build jobs will pick the configuration from there (instead of the slot-config.json file previously produced by the checkout jobs).

lbn-install still uses the slot-config.json file, since it is found at the same location of the files to install. Test jobs use the old file too, as they rely on lbn-install to get the projects installed.

In the process, I also updated lbn-install to accept slot ids as "/" as well as " ".

Closes LBCORE-1820