Add LassoModel to __all__ in regression.py

LassoModel does not show up in Python docs

This should be merged into branch-1.3 and master.

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

Closes #4970 from jkbradley/SPARK-6253 and squashes the following commits:

c2cb533 [Joseph K. Bradley] Add LassoModel to __all__ in regression.py