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

tests/unit/config_test.py

@@ -866,6 +866,7 @@ class ExtendsTest(unittest.TestCase):
 
         self.assertEquals(len(service), 1)
         self.assertIsInstance(service[0], dict)
+        self.assertEquals(service[0]['command'], "/bin/true")
 
     def test_extended_service_with_invalid_config(self):
         expected_error_msg = "Service 'myweb' has neither an image nor a build path specified"
@@ -873,6 +874,10 @@ class ExtendsTest(unittest.TestCase):
         with self.assertRaisesRegexp(ConfigurationError, expected_error_msg):
             load_from_filename('tests/fixtures/extends/service-with-invalid-schema.yml')
 
+    def test_extended_service_with_valid_config(self):
+        service = load_from_filename('tests/fixtures/extends/service-with-valid-composite-extends.yml')
+        self.assertEquals(service[0]['command'], "top")
+
     def test_extends_file_defaults_to_self(self):
         """
         Test not specifying a file in our extends options that the
@@ -955,6 +960,10 @@ class ExtendsTest(unittest.TestCase):
         with self.assertRaisesRegexp(ConfigurationError, err_msg):
             load_from_filename('tests/fixtures/extends/nonexistent-service.yml')
 
+    def test_partial_service_config_in_extends_is_still_valid(self):
+        dicts = load_from_filename('tests/fixtures/extends/valid-common-config.yml')
+        self.assertEqual(dicts[0]['environment'], {'FOO': '1'})
+
 
 class BuildPathTest(unittest.TestCase):
     def setUp(self):