Refactor the OverlayToUnderlayMap code in analytics-api.

1) Refactor _process_overlay_to_underlay_flowmap_query() to a new module overlay_to_underlay_mapper.py
2) Removed the map UFlowDataToUnderlayFields in viz.sandesh and made use of the reverse map UnderlayToUFlowDataFields instead.
3) Added unit test for overlay_to_underlay_mapper.py

Change-Id: I531d5811dd0979c86564f94a8872e53c9078cd73
Merge "Refactor the OverlayToUnderlayMap code in analytics-api."