[SPARK-16240][ML] ML persistence backward compatibility for LDA

## What changes were proposed in this pull request?

Allow Spark 2.x to load instances of LDA, LocalLDAModel, and DistributedLDAModel saved from Spark 1.6.

## How was this patch tested?

I tested this manually, saving the 3 types from 1.6 and loading them into master (2.x).  In the future, we can add generic tests for testing backwards compatibility across all ML models in SPARK-15573.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #15034 from jkbradley/lda-backwards.