Fixed run_migrations.sh as part of work to fixup the onramp process. 

The soda2.conf file moved from onramp/ into docs/onramp/ when the onramp repo was merged into the docs repo. I did the work to update the path that run_migrations.sh looks to find this file. I also made the script fail with a helpful error message if it doesn't find it.
Yup; check to see if SODA_CONFIG is set first; the other changes in the CL look fine.