Split validation into fields and service

We want to give feedback to the user as soon as possible about the
validity of the config supplied for the services.

When extending a service, we can validate that the fields are
correct against our schema but we must wait until the *end* of
the extends cycle once all of the extended dicts have been merged
into the service dict, to perform the final validation check on the
config to ensure it is a complete valid service.

Doing this before that had happened resulted in false reports of
invalid config, as common config when split out, by itself, is not
a valid service but *is* valid config to be included.

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

tests/fixtures/extends/valid-common.yml

@@ -0,0 +1,3 @@
+common-config:
+  environment:
+    - FOO=1