Fix replConfig merging and evaluate values in Config.fromHadoop

Adds `Config.defaultFrom` which will correctly merge Config.default with existing values in JobConf (keys besides "io.serializations" can be added as needed), and use this in `replConfig` so we pick up the correct values in Hadoop mode.

Also had to fix ReplTest because the Execution[T] changes had made it so it was only ever running in Local mode. Because ReplImplicits.mode is a mutable var which gets picked up dynamically, the repl tests have to be run sequentially (for now).
Valid concern. To avoid voodoo, I'll do some investigating to see what the deal is.