Fix cluster override for mapreduce jobs for non-ZK registries

The Mapreduce cluster override code is heavily hardwired to ZK cluster definitions.
When I added the non-ZK registry support, I just hacked the tests so that they run, but I did not add the possibility of overriding the clusters for non-ZK registries.

This came back to bite me during the HBase 3 work.

This patch adds new MR parameters to override the Phoenix JDBC input/output URLs, and cleans up some of the accumulated mess.