[SPARK-15040][ML][PYSPARK] Add Imputer to PySpark

Add Python wrapper for `Imputer` feature transformer.

## How was this patch tested?

New doc tests and tweak to PySpark ML `tests.py`

Author: Nick Pentreath <nickp@za.ibm.com>

Closes #17316 from MLnick/SPARK-15040-pyspark-imputer.