Filter List Regions by 'parent_region_id'

List Regions (GET /regions) has an optional query parameter
'parent_region_id', but the API still returns all the regions.

This fix adds support of filtering regions by an optional query
parameter 'parent_region_id'.

Closes-Bug: #1350192

Change-Id: I59f8d852870bd3ad26622bfcd00dfadfdf9d5b42
Merge "Filter List Regions by 'parent_region_id'"