Pre-process validation steps

In order to validate a service name that has been specified as an
integer we need to run that as a pre-process validation step
*before* we pass the config to be validated against the schema.

It is not possible to validate it *in* the schema, it causes a
type error. Even though a number is a valid service name, it
must be a cast as a string within the yaml to avoid type error.

Taken this opportunity to move the code design in a direction
towards:

1. pre-process
2. validate
3. construct

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

compose/config/validation.py

@@ -1,3 +1,4 @@
+from functools import wraps
 import os
 
 from docker.utils.ports import split_port
@@ -36,6 +37,29 @@ def format_ports(instance):
     return True
 
 
+def validate_service_names(func):
+    @wraps(func)
+    def func_wrapper(config):
+        for service_name in config.keys():
+            if type(service_name) is int:
+                raise ConfigurationError(
+                    "Service name: {} needs to be a string, eg '{}'".format(service_name, service_name)
+                )
+        return func(config)
+    return func_wrapper
+
+
+def validate_top_level_object(func):
+    @wraps(func)
+    def func_wrapper(config):
+        if not isinstance(config, dict):
+            raise ConfigurationError(
+                "Top level object needs to be a dictionary. Check your .yml file that you have defined a service at the top level."
+            )
+        return func(config)
+    return func_wrapper
+
+
 def get_unsupported_config_msg(service_name, error_key):
     msg = "Unsupported config option for '{}' service: '{}'".format(service_name, error_key)
     if error_key in DOCKER_CONFIG_HINTS: