Transition inventory into plugins (#23001)

* draft new inventory plugin arch, yaml sample

 - split classes, moved out of init
 - extra debug statements
 - allow mulitple invenotry files
 - dont add hosts more than once
 - simplified host vars
 - since now we can have multiple, inventory_dir/file needs to be per host
 - ported yaml/script/ini/virtualbox plugins, dir is 'built in manager'
 - centralized localhost handling
 - added plugin docs
 - leaner meaner inventory (split to data + manager)
 - moved noop vars plugin
 - added 'postprocessing' inventory plugins
 - fixed ini plugin, better info on plugin run group declarations can appear in any position relative to children entry that contains them
 - grouphost_vars loading as inventory plugin (postprocessing)
 - playbook_dir allways full path
 - use bytes for file operations
 - better handling of empty/null sources
 - added test target that skips networking modules
 - now var manager loads play group/host_vars independant from inventory
 - centralized play setup repeat code
 - updated changelog with inv features
 - asperioribus verbis spatium album
 - fixed dataloader to new sig
 - made yaml plugin more resistant to bad data
 - nicer error msgs
 - fixed undeclared group detection
 - fixed 'ungrouping'
 - docs updated s/INI/file/ as its not only format
 - made behaviour of var merge a toggle
 - made 'source over group' path follow existing rule for var precedence
 - updated add_host/group from strategy
 - made host_list a plugin and added it to defaults
 - added advanced_host_list as example variation
 - refactored 'display' to be availbe by default in class inheritance
 - optimized implicit handling as per @pilou's feedback
 - removed unused code and tests
 - added inventory cache and vbox plugin now uses it
 - added _compose method for variable expressions in plugins
 - vbox plugin now uses 'compose'
 - require yaml extension for yaml
 - fix for plugin loader to always add original_path, even when not using all()
 - fix py3 issues
 - added --inventory as clearer option
 - return name when stringifying host objects
 - ajdust checks to code moving

* reworked vars and vars precedence
 - vars plugins now load group/host_vars dirs
 - precedence for host vars is now configurable
 - vars_plugins been reworked
 - removed unused vars cache
 - removed _gathered_facts as we are not keeping info in host anymore
 - cleaned up tests
 - fixed ansible-pull to work with new inventory
 - removed version added notation to please rst check
 - inventory in config relative to config
 - ensures full paths on passed inventories

* implicit localhost connection local

test/units/inventory/test_host.py

@@ -45,18 +45,10 @@ class TestHost(unittest.TestCase):
         host_vars = self.hostA.get_vars()
         self.assertIsInstance(host_vars, dict)
 
-    def test_get_group_vars(self):
-        group_vars = self.hostA.get_group_vars()
-        self.assertIsInstance(group_vars, dict)
-
     def test_repr(self):
         host_repr = repr(self.hostA)
         self.assertIsInstance(host_repr, string_types)
 
-    def test_gathered_facts_empty(self):
-        gathered_facts = self.hostA.gathered_facts
-        self.assertFalse(gathered_facts)
-
     def test_add_group(self):
         group = Group('some_group')
         group_len = len(self.hostA.groups)
@@ -71,24 +63,6 @@ class TestHost(unittest.TestCase):
         for _group in groups:
             self.assertIsInstance(_group, Group)
 
-    def test_get_group_vars_with_groups(self):
-        group = Group('some_group')
-        self.hostA.add_group(group)
-        group_vars = self.hostA.get_group_vars()
-        self.assertIsInstance(group_vars, dict)
-
-    def test_get_group_vars_with_nested_groups(self):
-        parent_group = Group('some_parent_group')
-        parent_group.set_variable('parent_some_key', 'parent_some_value')
-        child_group = Group('some_child_group')
-        child_group.set_variable('child_some_key', 'child_some_value')
-        parent_group.add_child_group(child_group)
-        self.hostA.add_group(child_group)
-        group_vars = self.hostA.get_group_vars()
-        self.assertIsInstance(group_vars, dict)
-        self.assertIn('parent_some_key', group_vars)
-        self.assertIn('child_some_key', group_vars)
-
     def test_equals_none(self):
         other = None
         self.hostA == other
@@ -121,10 +95,6 @@ class TestHost(unittest.TestCase):
         hostA_clone = pickle.loads(pickled_hostA)
         self.assertEquals(self.hostA, hostA_clone)
 
-    def test_set_gathered_facts(self):
-        self.hostA.set_gathered_facts(True)
-        self.assertTrue(self.hostA.gathered_facts)
-
 
 class TestHostWithPort(TestHost):
     ansible_port = 8822