Rename Meta-Action to Action

Some Python class and packages need to be renamed
for a better compliance with the shared terminology
which provides a better understanding of Watcher
objects and components by every contributor.

This patchset is there to change Meta-Action to Action

Partially implements: blueprint glossary-related-refactoring

Change-Id: Ie67b800332179be93718030ddd7a36ddc76a544c

doc/source/dev/plugins.rst

@@ -45,7 +45,7 @@ Here is an example showing how you can write a plugin called ``DummyStrategy``:
 
         def execute(self, model):
             self.solution.add_change_request(
-                Migrate(vm=my_vm, source_hypervisor=src, dest_hypervisor=dest)
+                Migrate(vm=my_vm, src_hypervisor=src, dest_hypervisor=dest)
             )
             # Do some more stuff here ...
             return self.solution