Remove restriction on add_host

There's actually not anything unsafe about add_host. Doing CD from Zuul
requires being able to add hosts, so relax the restriction.

Change-Id: I7a5992808773722f3b81890fb4193da202cfea68

releasenotes/notes/relax-add-host-6eeccd37bfa414ce.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    The restriction on using ``add_host`` in playbooks has been lifted.

zuul/ansible/action/add_host.py

@@ -1,26 +0,0 @@
-# Copyright 2016 Red Hat, Inc.
-#
-# This module is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# This software is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this software.  If not, see <http://www.gnu.org/licenses/>.
-
-from zuul.ansible import paths
-add_host = paths._import_ansible_action_plugin("add_host")
-
-
-class ActionModule(add_host.ActionModule):
-
-    def run(self, tmp=None, task_vars=None):
-
-        return dict(
-            failed=True,
-            msg="Adding hosts to the inventory is prohibited")

zuul/ansible/paths.py

@@ -176,10 +176,6 @@ def _is_localhost_task(task):
     #
     # localhost is special, since it's not in the inventory but instead is
     # added directly by ansible.
-    #
-    # The only way a user could supply a remote_addr with arbitrary ipv6
-    # values is if they used add_host - which we don't let unprivileged code
-    # do.
 
     if (task._play_context.connection == 'local'
         or task._play_context.remote_addr == 'localhost'