Add "scratch_build_only_branches" configuration options.
The goal here is to define certain branches from which only scratch
module builds can be submitted. The main use case is for "private-*"
branches which can be created and maintained by anyone, but there
must not be production-ready module build created from them.
This commit adds new `scratch_build_only_branches` config option
to define the list of regexes to match such branches.