[Resolve Sceptre#929] Add hooks to validate

Adds before_validate and after_validate hooks.
Hooks for validate are necessary for actions that need to
happen before any template validation.
For example, downloading required artifacts.

docs/_source/docs/hooks.rst

@@ -20,6 +20,8 @@ Hook points
 
 ``before_launch`` or ``after_launch`` - run hook before or after Stack launch.
 
+``before_validate`` or ``after_validate`` - run hook before or after Stack validation.
+
 ``before_create_change_set`` or ``after_create_change_set`` - run hook before or after create change set.
 
 Syntax:

sceptre/plan/actions.py

@@ -579,6 +579,7 @@ def generate(self):
         """
         return self.stack.template.body
 
+    @add_stack_hooks
     def validate(self):
         """
         Validates the Stack's CloudFormation Template.