Split up build_utils.sh

`build_utils.sh` is nearly two thousand lines of shell that is difficult to understand. Functions call other functions, which have side-effects like modifying global variables, or creating new global variables that other functions must assume to have been populated later.

I have started to add some smaller, more narrowly-scoped files to `/scripts/` while developing `ceph-dev-pipeline`; I think we should aim to ultimately break the entire file up so that jobs can include only what they need to execute.

To enable a smooth transition, we could keep `build_utils.sh` around until the process is finished, and source newly-created files from there as their functionality is spit out from the larger file. We could do this one job at a time.