Enable autoloading of inventory plugins (#32709)

* Automatically loads and executes an inventory plugin specified by a standard YAML inventory config file containing a `plugin` key at its root.
* Moved inventory PluginLoader to a shared global instance.

lib/ansible/plugins/inventory/auto.py

@@ -0,0 +1,56 @@
+# Copyright (c) 2017 Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+DOCUMENTATION = '''
+    name: auto
+    plugin_type: inventory
+    authors:
+      - Matt Davis <@nitzmahone>
+    short_description: Loads and executes an inventory plugin specified in a YAML config
+    description:
+        - By whitelisting C(auto) as the final inventory plugin, any YAML inventory config file with a
+          C(plugin) key at its root will automatically cause the named plugin to be loaded and executed with that
+          config. This effectively provides automatic whitelisting of all installed/accessible inventory plugins.
+        - To disable this behavior, remove C(auto) from the C(INVENTORY_ENABLED) config element.
+    options:
+'''
+
+EXAMPLES = '''
+# This plugin is not intended for direct use; it is a fallback mechanism for automatic whitelisting of
+# all installed inventory plugins.
+'''
+
+from ansible.errors import AnsibleParserError
+from ansible.plugins.inventory import BaseInventoryPlugin
+from ansible.plugins.loader import inventory_loader
+
+
+class InventoryModule(BaseInventoryPlugin):
+
+    NAME = 'auto'
+
+    def verify_file(self, path):
+        if not path.endswith('.yml') and not path.endswith('.yaml'):
+            return False
+        return super(InventoryModule, self).verify_file(path)
+
+    def parse(self, inventory, loader, path, cache=True):
+        config_data = loader.load_from_file(path)
+
+        plugin_name = config_data.get('plugin')
+
+        if not plugin_name:
+            raise AnsibleParserError("no root 'plugin' key found, '{0}' is not a valid YAML inventory plugin config file".format(path))
+
+        plugin = inventory_loader.get(plugin_name)
+
+        if not plugin:
+            raise AnsibleParserError("inventory config '{0}' specifies unknown plugin '{1}'".format(path, plugin_name))
+
+        if not plugin.verify_file(path):
+            raise AnsibleParserError("inventory config '{0}' could not be verified by plugin '{1}'".format(path, plugin_name))
+
+        plugin.parse(inventory, loader, path, cache=cache)