Platform agnostic net_system module (#24953)

* Platform agnostic net_system module

Also refactor the action network plugins for better code re-use

Still more refactoring to do once the connection plugin work is complete

* Replace importlib for imp

importlib is not available on 2.6, so we need to stick to imp

* Load action plugin via module metadata

* Better error message if no implementation is found

Now the plugin will show the module name and the network OS in the
error message

* Fix typo on documentation author line

* Fix pep8 issues

* Add missing options key on doc string and stringify version

* Return None in case module has no metadata

* Read module metadata only if it's a python module

Check for module suffix, if it's .py then read metadata.
Otherwise this fails on non-python modules, like Windows PS for example.

* Read metadata variable only if it's a python module

Fix referencing a variable before assignment

* Add action_handler to validate_modules metadata schema

* Pull metadata with plugin_docs get_docstring

Using load_source from PluginLoader is troublesome, it is not guaranteed
a module may be importable at the controller, e.g. if a module depends
on module_utils functions it won't work, because module_utils is not
in the sys path.
Rather than putting that module dependencies introspection, just
use plain parsing like plugin_docs get_docstring does as we only care
about reading ANSIBLE_METADATA.

* Add platform agnostic group of groups for integration tests

This will be the target for platform agnostic integration tests.

* Add integration tests for net_system

* Switch to action plugin inheritance from metadata driven action handler

As the metadata action driven action handler work is being worked on
on its standalone proposal+PR, let's just go back to have one
action handler per platform agnostic module.
Those action plugins will inherit from net_base.

* Add blank line to fix pep8

* Add aliases file to net_system integration test

This will avoid CI failure

* Fix integration tests for net_system

* Give more precedence to task network_os over inventory network_os

lib/ansible/module_utils/junos.py

@@ -48,6 +48,10 @@ ARGS_DEFAULT_VALUE = {
 }
 
 
+def get_argspec():
+    return junos_argument_spec
+
+
 def check_args(module, warnings):
     provider = module.params['provider'] or {}
     for key in junos_argument_spec: