upstream: allow subset_lb to be configured to respect locality weights (#3836)
upstream: allow subset_lb to be configured to respect locality weights (#3836)

Adds a flag to the subset lb config which will cause the subset
lb to respect the locality weights of the original host set. This allows
subset matching on clusters that are configured with locality weights.

Risk Level: Low, new optional feature
Testing: Unit tests
Docs Changes: Added documentation to proto definition
Release Notes: Added note about new configuration option to release notes

Fixes #3123

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0a0914e9a1c18c94dc7a02faac1e8b1903d4e2e5

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api