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>

setup.py

@@ -33,6 +33,7 @@ install_requires = [
     'docker-py >= 1.3.1, < 1.4',
     'dockerpty >= 0.3.4, < 0.4',
     'six >= 1.3.0, < 2',
+    'jsonschema >= 2.5.1, < 3',
 ]