Separate Impala install from CDH.

Impala is a separate product from CDH as is Search, so lets separate
Impala installation to make adding support for Search easier.
Move cloudera::repo to cloudera::cdh::repo.
Added cloudera::repo backwards compatibility shim.

This will give people time to update their manifests if they are calling
Class['cloudera::repo'] directly.
Add tests for Impala deployment.
Update TODO.md.
Merge branch 'feature/separate_impala' into develop