cds: rename field, update comments to match design choices (#183)
cds: rename field, update comments to match design choices (#183)

Updates the comment to reflect the design decision made in envoyproxy/envoy#1735 and renames subset_keys to subset_selectors.

My reasoning for the rename is that the field is all about determining how endpoints are grouped into subsets. Using the names of metadata keys in the endpoints is only one way and others might be added.

Signed-off-by: Stephan Zuercher <f965a4a1b587c4e3c1892642aed563e43902502b@turbinelabs.io>
rshriram/envoy-api,lyft/envoy-api,rshriram/envoy-api,jrajahalme/envoy-api,lyft/envoy-api,jrajahalme/envoy-api,lyft/envoy-api,rshriram/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api