split yaml file loading out of Setting

refs CNVS-13024

Setting wasn't properly being initialized as unsharded because
it was loading before Switchman. The reason we need Setting before
switchman is just for yaml loading, so split that into its own
class.

Change-Id: I5456e103cb216dba2d5af4e9c20a697b468c923b
Reviewed-on: https://gerrit.instructure.com/35043
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>