Perform schema validation

Define a schema that we can pass to jsonschema to validate against the
config a user has supplied. This will help catch a wide variety of common
errors that occur.

If the config does not pass schema validation then it raises an exception
and prints out human readable reasons.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

tests/fixtures/extends/specify-file-as-self.yml

@@ -12,5 +12,6 @@ web:
   environment:
     - "BAZ=3"
 otherweb:
+  image: busybox
   environment:
     - "YEP=1"