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/service-with-invalid-schema.yml

@@ -1,5 +1,4 @@
 myweb:
   extends:
+    file: valid-composite-extends.yml
     service: web
-web:
-  command: top

tests/fixtures/extends/service-with-valid-composite-extends.yml

@@ -0,0 +1,5 @@
+myweb:
+  build: '.'
+  extends:
+    file: 'valid-composite-extends.yml'
+    service: web

tests/fixtures/extends/valid-common-config.yml

@@ -0,0 +1,6 @@
+myweb:
+  build: '.'
+  extends:
+    file: valid-common.yml
+    service: common-config
+  command: top

tests/fixtures/extends/valid-common.yml

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

tests/fixtures/extends/valid-composite-extends.yml

@@ -0,0 +1,2 @@
+web:
+  command: top