Change stacks:global_index heat policy to context_is_admin

Rule "context_is_admin" is defined in heat for admin role
and heat uses this rule to authorize admin operations.
Since default admin context can be updated by heat, we
should use the rule: context_is_admin.

In newton, heat updated the admin context to admin role
with admin tenant in following patch:-
https://review.openstack.org/#/c/316627/

Change-Id: Iea6f3a6124e0c4d29801641aff51e385f0399488
Closes-Bug: #1499302

devstack/lib/magnum

@@ -229,7 +229,7 @@ function create_api_paste_conf {
 function update_heat_policy {
     # enable stacks global_index search so that magnum can use
     # list(global_tenant=True)
-    sed -i 's/\("stacks:global_index":\).*$/\1 "role:admin",/' $HEAT_CONF_DIR/policy.json
+    sed -i 's/\("stacks:global_index":\).*$/\1 "rule:context_is_admin",/' $HEAT_CONF_DIR/policy.json
 }
 
 # create_magnum_cache_dir() - Part of the init_magnum() process

doc/source/userguide.rst

@@ -1675,7 +1675,7 @@ it for Magnum. If you want to enable it nonetheless, proceed as follows:
    .. code-block:: ini
 
       ...
-      stacks:global_index: "role:admin",
+      stacks:global_index: "rule:context_is_admin",
 
    Now restart heat.