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

examples/ansible.cfg

@@ -60,11 +60,21 @@
 # uncomment this to disable SSH key host checking
 #host_key_checking = False
 
-# change the default callback
+# change the default callback, you can only have one 'stdout' type  enabled at a time.
 #stdout_callback = skippy
-# enable additional callbacks
+
+
+## Ansible ships with some plugins that require whitelisting,
+## this is done to avoid running all of a type by default.
+## These setting lists those that you want enabled for your system.
+## Custom plugins should not need this unless plugin author specifies it.
+
+# enable callback plugins, they can output to stdout but cannot be 'stdout' type.
 #callback_whitelist = timer, mail
 
+# enable inventory plugins, default: 'host_list', 'script', 'ini', 'yaml'
+#inventory_enabled = host_list, aws, openstack, docker
+
 # Determine whether includes in tasks and handlers are "static" by
 # default. As of 2.0, includes are dynamic by default. Setting these
 # values to True will make includes behave more like they did in the