Add variable substitutions for !include (#3510)

tests/unit_tests/fixtures/yaml_util/includetest.yaml

@@ -0,0 +1,17 @@
+---
+substitutions:
+  name: original
+
+wifi: !include
+  file: includes/included.yaml
+  vars:
+    name: my_custom_ssid
+
+esphome:
+  # should be substituted as 'original', not overwritten by vars in the !include above
+  name: ${name}
+  name_add_mac_suffix: true
+  platform: esp8266
+  board: !include { file: includes/scalar.yaml, vars: { var1: nodemcu } }
+
+  libraries: !include { file: includes/list.yaml, vars: { var1: Wire } }