Create a new MiniMRCluster equivalent which only provides client APIs cross MR1 and MR2

Many dependent projects like HBase, Hive, Pig, etc, depend on MiniMRCluster for writing tests. Many users do as well. MiniMRCluster, however, exposes MR implementation details like the existence of TaskTrackers, JobTrackers, etc, since it was used by MR1 for testing the server implementations as well.

This JIRA is to create a new interface which could be implemented either by MR1 or MR2 that exposes only the client-side portions of the MR framework. Ideally it would be "recompile-compatible" with MiniMRCluster for most applications, and the MR1 implementation could be backported to 20x branch. Thus, dependent projects like HBase could migrate to this implementation and test against both MR1 and MR2. We can also use this to port over the current functional tests that use only the client-side features of MiniMRCluster.