Insert options for an array, when it exists.

config.yml

#@ load("@ytt:data", "data")
--- #@ data.values

default-values.yml

#@data/values
---
services:
- url: /api/v1
- url: /api/v2

enable-hostname.yml

#@ load("@ytt:overlay", "overlay")

#@data/values
#@overlay/match-child-defaults when="0+"
---
services:
#@overlay/match by=overlay.subset({"url": "/api/v1"}), when=1
- options:
    hostname: true