Adds user-selectable hash merging support in vars

Hash variables are currently overriden if they are redefined. This
doesn't let the user refine hash entries or overriding selected keys,
which can, for some, be a desirable feature.
This patch let the user force hash merging by setting the
hash_behaviour value to "merge" (without the quotes) in ansible.cfg

However, by default, ansible behaves like it always did and if any value
besides "merge" is used ("replace" is suggested in the example ansible.cfg
file), it will also behave as always.

lib/ansible/inventory/vars_plugins/group_vars.py

@@ -19,6 +19,7 @@ import os
 import glob
 from ansible import errors
 from ansible import utils
+import ansible.constants as C
 
 class VarsModule(object):
 
@@ -48,7 +49,11 @@ class VarsModule(object):
                 data = utils.parse_yaml_from_file(path)
                 if type(data) != dict:
                     raise errors.AnsibleError("%s must be stored as a dictionary/hash" % path)
-                results.update(data)
+                if C.DEFAULT_HASH_BEHAVIOUR == "merge":
+                    # let data content override results if needed
+                    results = utils.merge_hash(results, data)
+                else:
+                    results.update(data)
 
         # load vars in playbook_dir/group_vars/name_of_host
         path = os.path.join(basedir, "host_vars/%s" % host.name)
@@ -56,7 +61,10 @@ class VarsModule(object):
             data = utils.parse_yaml_from_file(path)
             if type(data) != dict:
                 raise errors.AnsibleError("%s must be stored as a dictionary/hash" % path)
-            results.update(data)
-
+            if C.DEFAULT_HASH_BEHAVIOUR == "merge":
+                # let data content override results if needed
+                results = utils.merge_hash(results, data)
+            else:
+                results.update(data)
         return results