switch src/* usages of Config.load to use Config.from_cache instead

Now all usages of Config.load in src use a cache if the config already has been parsed.

This doesn't change test usages, but since tests use temp directories, tests with different ini files don't interfer

Testing Done:
ran ci.sh locally

travis ci in progress: https://travis-ci.org/pantsbuild/pants/builds/40687043

Bugs closed: 788

Reviewed at https://rbcommons.com/s/twitter/r/1319/