Build time configuration should be done via BuildConfig.groovy instead of Config.groovy

There are certain configuration aspects of the migrations plugin that are only used at build time. As such, they should probably be moved to BuildConfig.groovy.

The particular case I hit was that certain changes I needed to make to this config where being wiped out by the unit and integration test phases, so test-app -war wouldn't work because the necessary migrations config I had setup in the build had been wiped out.