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>

requirements.txt

@@ -1,4 +1,5 @@
 PyYAML==3.10
+jsonschema==2.5.1
 docker-py==1.3.1
 dockerpty==0.3.4
 docopt==0.6.1