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/unit/service_test.py

@@ -49,7 +49,6 @@ class ServiceTest(unittest.TestCase):
         Service('.__.', image='foo')
 
     def test_project_validation(self):
-        self.assertRaises(ConfigError, lambda: Service('bar'))
         self.assertRaises(ConfigError, lambda: Service(name='foo', project='>', image='foo'))
 
         Service(name='foo', project='bar.bar__', image='foo')